Uninvited Guests in the Aquarium

Uninvited guests in the aquarium.

There are many aquarium parasites. The fact that the problem of eradicating all sorts of nastiness periodically arises among aquarists is neither news nor a sensation. These uninvited guests are extremely resilient. As you can see, many chemical and even electrochemical methods of combating Hydra have been devised, but all of them have numerous drawbacks. Disruption of the biofilter, the susceptibility of many fishes to the drugs used, and the need for large water changes after treatment are hardly likely to make anyone happy.

Planaria (Planaria).

One of the common problems in a freshwater invertebrate aquarium is planarian worms. Brought into the aquarium with untreated substrate, aquatic plants, snails, live, and even frozen foods, they become a scourge for aquarium inhabitants.

Planarians love protein food: they eat shrimp eggs, young fish and shrimp, small invertebrates, food for fish and shrimp, and some snails. They can also attack adult shrimp during molting or crawl under the carapace of crayfish, clogging their gills, causing suffocation, and then feasting on the victim. Neither young crustaceans nor adults, especially after molting, can feel safe. If left to chance, the entire decapod population may be destroyed in a very short time. But even if you do not aim to breed crayfish and shrimp, stay alert. In search of food, agile worms crawl under the carapace of crustaceans and clog their gills, causing suffocation. Some aquarists may not guess the true reason for the absence of young shrimp despite females visibly carrying eggs. If female shrimp regularly carry eggs, but there is no offspring, check the aquarium for planarians!

Planarians belong to flatworms. Small whitish-pinkish worms with a triangular anterior part. Average size ranges from 0.3 to 1.0 centimeters. They have two eyes, a triangular head, and various colorations. In home aquariums, white-milky (Dendrocoelum lacteum), mourning (Planaria lugubris), or brown (Planaria torva) planarians are mostly found. The body of planarians is covered with cilia, and their movements look smooth. They move closer to the water surface; unlike hydras, they are active at twilight and night, hiding in the substrate, under stones, and among plants during the day, making them difficult to notice, especially brown or mourning planarians. White-milky ones reveal themselves faster due to their coloration. They are very flexible and capable of slipping into even a tiny crevice. The body stretches and contracts. If disturbed, they curl into a ring and fall onto the substrate. The worms are covered with a specific mucus, so fish, with a few exceptions, do not eat them.

Planarians are extraordinarily resilient. A planarian cut into parts regenerates, and eventually, several new planarians grow from the pieces. The worms are capable of astonishing regeneration: divided into more than two hundred parts, they retain the ability to fully recover from each part. Upon entering adverse conditions, they lose mobility, turning into a shapeless mass, but the pharynx, which can live independently, detaches from the body and enters the environment, continuing to move.

The main cause of planarian reproduction is overfeeding aquarium inhabitants. The planarian problem often arises where large fish feed on food that disintegrates in their mouths. After that, a rain of food particles flies out of the gills and falls to the bottom. These particles are too small to be of interest to large fish. In such cases, the issue may not be the quantity, but the type of food, which is the real problem. A possible solution is to change the diet or keep fish in the aquarium that have the habit of digging in the substrate and cleaning up everything that has fallen from above.

Combating planarians: How to get rid of planarians? Eradicating planarians is very difficult. The only measures to combat planarians consist of removing them from the aquarium with a net or siphon, using chemical preparations safe for hydrobionts, and preventing their appearance by avoiding overfeeding fish and shrimp.

Methods of combating this resilient predator are based on its highly developed sense of smell. A small amount of scraped beef is placed in a gauze bag and lowered on a fishing line into the aquarium, near plants or the substrate. The aquarium is darkened. Planarians gather on the bait quite quickly. The bag is removed with a net and lowered into boiling water. The procedure is repeated at intervals of a few days. Unfortunately, these measures are of low efficiency. Worms that escaped capture and eggs left in the aquarium will reduce your efforts to zero in the shortest possible time. In addition, it is inconvenient in terms of aquarium organic pollution, as such “operations” must be carried out for more than one day.

One of the methods to combat planarians is a thorough inspection and treatment of aquarium plants, stones, and substrate to destroy the eggs of these worms. On the smooth surface of stones and the underside of leaves, small sacs with planarian eggs can be seen and washed off. The substrate should be removed and boiled. But even this method is labor-intensive and does not give a 100% guarantee of planarian eradication.

There are other ways to destroy planarians: passing an electrical charge (12 V) through the water, increasing the water temperature in the aquarium, and treating with various chemicals, but these methods are unsafe for snails and shrimp. Raising the water temperature to 30-32°C, which many tropical fish can withstand (provided sufficient aeration), is also fatal to planarians. Chemical methods of combating planarians are also acceptable. Copper sulfate (1.5 mg per 1 liter of water) and ammonium nitrate (25-50 mg per 1 liter of water with double treatment) are used.

Some fish species kept for 2-3 days without food, such as young gouramis, paradise fish, bettas, young convict cichlids, jewel cichlids, and plecos, eat planarians. But in a shrimp tank, you will have to work hard.

To date, the simplest and most effective method is the use of preparations containing fenbendazole. This active substance is harmless to snails and crustaceans, but lethal to planarians and other flatworms. Recommended dosage: 0.2-0.4 grams of fenbendazole per 100 liters of water. After 24-48 hours, all planarians perish. The effect of using fenbendazole in the form of a suspension occurs faster than dissolving powder in water yourself, as fenbendazole is poorly soluble in water. Dead worms can be found sticking to the aquarium glass. Mechanical cleaning and a water change will put the final period in the fight against these parasites.

Hydra (Hydra).

Another common parasite is Hydra. Hydra enters the aquarium along with live food, newly acquired plants or duckweed, and any object of natural origin. You cannot even establish the very fact of introducing eggs or microscopic hydras with substrate, driftwood, plants, live food, or even milligrams of water in which shrimp, snails, fish, or live food were purchased. If you carefully inspect plants removed from a body of water, you can notice small light brown mucous lumps on them. These are hydras that have shriveled when pulled out of the water. Before placing plants in the aquarium, they must be disinfected. Even if hydras are not visibly present in the aquarium, they can be found by examining any piece of driftwood or stone under a microscope. Adult hydras die in winter, so at this time of year, it is impossible to introduce hydra into the aquarium with food. But in summer, it often gets into the aquarium along with caught crustaceans. If the food is not carefully inspected, hydras can enter the aquarium when feeding fish.

Hydra belongs to the group of coelenterate aquatic invertebrates, ranging in size from 1 to 20 mm. Its body is a stalk-stem by which it attaches to any surfaces in the aquarium – glass, substrate, driftwood, plants, and even snail egg clutches. In the body of a hydra, one can distinguish a longer main part and a thinner stem, the base of which, the sole, is attached to the substrate. The mouth opening is surrounded by tentacles. The number of tentacles, their length, and position relative to the body vary in different species of hydra. Inside the hydra’s body is the main organ that constitutes its essence – the stomach.

The body and tentacles are covered with stinging cells, on the surface of which there is a sensitive hair. When irritated by a passing victim, a stinging thread is ejected outward from the stinging cells, entangling the victim, piercing it, and releasing poison that paralyzes the victim. Hydra uses its tentacles to capture prey. The long tentacles crowning the hydra’s body are in constant motion, capturing numerous small, sometimes invisible to the eye, creatures from the water and bringing them to the mouth at the end of the hydra’s body. It also feeds on tiny food particles suspended in the water column.

Hydra moves slowly, contracting and straightening its body and tentacles or “stepping head over heels”. As soon as something disturbs the unspooled animal, its body, resembling a long narrow cylinder, instantly becomes short and thick, and the tentacles contract sharply. Hydra can swim and even attach to the film on the water surface, live on it, and collect dry food, catching Artemia.

All of them are characterized by regeneration (the ability to restore lost organs). It can recreate itself from any piece of its body. The regeneration of a hydra from the smallest remnants of its body makes it practically immortal and unkillable. So rubbing it against the aquarium walls is useless. Hydras reproduce by budding (asexual variant) or by fertilizing an egg with a spermatozoon, resulting in the formation of an “egg” in the hydra’s body, which, after the death of the adult, waits for its time in the substrate or moss. Hydra thrives in dirty aquariums. The impetus for their rapid reproduction, actually when hydras become visible to the aquarist, is an excess of organics in the aquarium water. Once in the aquarium, hydras under abundant feeding quickly reproduce by budding, as a result of which plants, substrate, and aquarium glass are covered with a continuous fringe of their bodies. Hydras live in the aquarium for about a year.

As a true predator, hydra feeds only on animals, as it assimilates fats, proteins, and carbohydrates of animal origin only. Hydra hunts small invertebrates: cyclops, daphnia, oligochaetes, rotifers, trematode larvae. Fish fry or young shrimp can also fall into its death-dealing “paws”. It is known that hydra eats fish fry and small shrimp. In an aquarium where small fish live, and even more so where tiny fry are growing, hydras have no place. Once in a spawning or grow-out aquarium, a hydra, having rapidly reproduced by budding, will immediately deal with the fish fry. If you do not fight them, there will soon be no fry left at all, and small fish will suffer from chemical burns inflicted by the hydras with their stinging cells located in the tentacles. In principle, adult fish are of no interest to hydras. Nevertheless, swimming past the colony of these animals, fish touch their tentacles equipped with stinging cells, receive nettle-like burns, and suffer. For fry, such damage is often fatal. Hydra can also sting a snail or shrimp crawling or swimming past. In addition, spreading in large numbers, it affects the leaf blades of aquatic plants in such a way that the leaves’ ability to assimilate is lost.

Before the appearance of shrimp in our aquaculture (and the fashion for pure shrimp or shrimp-fish community aquariums), planarians and hydras were not a problem for the aquarist. Quite a few biological and chemical methods of combating these cute creatures are known, leaving no chance for populations to develop without permission. But in shrimp tanks, the situation is completely different. Because, considering the sensitivity of shrimp to metals and chemicals, in the event of “hostilities”, they will die faster than unwanted guests.

Combating Hydra: Of course, you always want to eradicate uninvited guests without harming other aquarium inhabitants, primarily shrimp, fish, and expensive snails. Therefore, salvation from hydras is predominantly sought among biological methods. First, hydra also has enemies that eat it. These are some fish: young and hungry black mollies, swordtails, gouramis (marbled and kissing), bettas, julidochromis, ancistrus, and apple snails (Ampullaria).

Hydra’s love for light is also used to combat it. It is noticed that hydra locates itself closer to the light source. A piece of glass is tightly pressed against the aquarium wall, and a light source (lamp or flashlight) is directed at that spot in the dark. As a result, overnight, hydras relocate to the trap glass, which is then pulled out of the water and scalded with boiling water. This remedy can rather be called controlling the number of hydras, as this method does not provide complete eradication.

In a fish-populated aquarium, the hydra population can be controlled by dissolving table salt in the water to make a 0.5% saline solution. Such a solution should be maintained for about a week, and then gradually get rid of the salt through multiple partial water changes. This method can only be used provided that all fish tolerate such salinity well.

Hydras also tolerate elevated temperatures poorly. The method of heating water in the aquarium is useful if it is possible to catch all valuable aquarium inhabitants and transplant them to another container. The water temperature in the aquarium is brought to 42°C and kept like that for 20-30 minutes, turning off the external filter or removing the filter medium from the internal filter. Then the water is allowed to cool. After that, fish and other hydrobionts can be reintroduced, and filtration restored. Most plants tolerate this procedure well.

You can destroy hydra with electric current. Two bundles of stripped copper wire immersed in water in opposite corners of the aquarium are connected to a 9-12 V DC source. Thoroughly insulate the wires from the aquarium frame. Just 3-4 activations for 1-2 minutes are enough to destroy hydras. Fish and plants feel normal during this. Catfishes, snails, and other bottom-dwelling inhabitants may die, so they must be removed for this time.

Among radical measures is the use of chemicals. There is a medicine used to treat helminthic infestations in a wide variety of animals, so you need to look for it in veterinary pharmacies. To destroy hydras, preparations whose active substance is fenbendazole are used: Panacur, Febtal, Flubenol, Flubentazole. However, you should pay attention that the preparation does not contain copper or any active substance other than fenbendazole, otherwise shrimp will not survive such treatment. The drug is introduced into the aquarium at the rate of 0.2 (up to 0.4 g) of the active substance per 100 liters of water and gradually increased. Before use, the powder should be ground as finely as possible. The drug is poorly soluble, so it makes sense to buy a ready-made suspension (usually 22.2%), or mix it thoroughly in a separate container. It is important to avoid overdose, as it is fraught with serious problems. Fish and plants do not react to Fenbendazole at “therapeutic doses”. But be careful, Flubenol kills many species of mollusks, and in addition, a disruption of the biological balance in the aquarium is possible after the therapy. The resulting suspension when poured into the aquarium will cause water turbidity and sediment on the substrate and objects in the aquarium. Undissolved particles of the medicine can be eaten by shrimp, but this is not scary. At the recommended dose, the medicine is harmless to shrimp; planarians and hydras die within 24-48 hours (in heavily polluted and fairly large aquariums, a massive water change and repeated administration of the drug after 48 hours is sometimes recommended). After this, be sure to make a 50% water change to remove dead planarians and toxins released by dying worms. Repeat the procedure a week later (to destroy worms that emerged from eggs during this period), and after that, your aquarium should ideally become free of planarians.

In empty aquariums without plants (for example, quarantine ones), with sufficient artificial aeration, you can quickly get rid of hydra using hydrogen peroxide. To do this, fish are caught from the aquarium, and 2 teaspoons of a 3% hydrogen peroxide solution per every ten liters of aquarium volume are added to the water. The peroxide is pre-diluted in a glass of water. However, to achieve the proper effect, a hydrogen peroxide solution at the rate of 40 ml per 100 liters of water must be poured daily for a week. The generated free oxygen causes such severe burns to hydras that they can no longer recover. After a few days, hydras die, after which the aquarium is cleaned, and the water is replaced. Shrimp and fish tolerate this procedure well, but plants not so much. The use of ozonizers is based on the same principle. For the complete destruction of hydra, two sessions of 10-15 minutes of intensive ozonation of the aquarium are enough; ozone will quickly kill hydras in the aquarium.

Some chemicals are also used successfully: • Ammonium sulfate at the rate of 5 grams per 100 liters of water, added once to the general aquarium, as it is harmless to fish at these doses. Hydras should die within 3-5 days; • Ammonium nitrate – 6 grams per 100 liters of water, three times, at intervals of three days. The drug is pre-dissolved in a separate container. Ammonium nitrate is more toxic, and when using it, fish fry and young fish must be removed from the aquarium. Large adult fish can be left. Hydras actively reproduce at this time. Then ammonium nitrate is added to the aquarium at the rate of 0.6 g of the preparation per 10 liters of water. Aeration is turned on for better mixing. The temperature is raised to 27-28°C and maintained until the end of the treatment course. Three days later, the initial dose of the same concentration is added. Hydras should die on the 5th-7th day. Do not completely change the water in the aquarium after destroying hydras. Ammonium nitrate in such concentrations is not harmful to fish, and even serves as fertilizer for plants.

German aquarist Fink recommends a chemical method to combat hydras. For 8-14 days, hydras are intensively fed with daphnia. During this time, they undergo intense budding. After a certain period of time, a state comes when they catch daphnia well, but hold them poorly, or do not catch them at all. Then, during feeding, 1 gram of ammonium nitrate, pre-dissolved in about 0.5 l of water, is added to the aquarium for every 10 liters of water and mixed thoroughly. It is also advisable to simultaneously increase the water temperature in the aquarium by about five degrees. After 2 days, add the same amount of ammonium nitrate again. Soon you can notice that the tentacles of the hydras hang powerlessly, later they are completely retracted, and the body shrivels, i.e., the depression stage is clearly evident. A few days later, hydras fall off the glass and plants and drop to the bottom. Fish, even fry, can be in the aquarium at this time, infusoria and other microorganisms remain unharmed. Water should not be changed, as ammonium nitrate is an excellent fertilizer, and plants will soon absorb it completely.

When using a copper sulfate solution (0.05 grams per 1 liter of water), hydra death occurs within an hour; 3-4 hours later, partial cleaning and a complete water change are performed. Fish are removed from the aquarium for the duration of these operations. Copper sulfate must be chemically pure or analytical grade. This event is complicated by a complete water change, i.e., actually recreating the aquarium from scratch. However, for special spawning and grow-out aquariums where there is no substrate and plants are usually kept in pots, this method is quite acceptable.

When applying any of the above methods, special attention must be paid to organic cleanliness in the aquarium: do not overfeed inhabitants, exclude feeding invertebrates with daphnia or artemia, and make water changes on time. To make the fight against hydra more effective, you need to use not one, but two or even three methods simultaneously.

Branded products for destroying hydras and planarians.

I would like to note right away that all these preparations contain copper chloride to one degree or another, and therefore their use can lead to the death of snails, various crayfish, and shrimp in the aquarium, and also affect characin fish, some catfishes, and loaches badly.

Genchem No-Planaria. An effective remedy for destroying planarians in the aquarium. Instructions for use: Dosage calculation for 50 l: First day – 1 measuring spoonful. Second day – 1 / 2 measuring spoonful. Third day – 1 / 4 measuring spoonful. Thus, we get 72 hours. In 72 hours you will notice a significant decrease in planarians. In case there are too many parasites, it is recommended not to reduce the dose on the following days. Planarians are dangerous for shrimp! Planarians crawl under the dorsal shield of crustaceans and further into the gills in search of food, which causes shrimp to die from suffocation. Planarians’ favorite delicacy is shrimp caviar. If you have planarians multiplying in your aquarium, you will notice a sharp death of aquarium shrimp.

With prolonged use, it kills nematodes and can decimate the snail population. Prolonged use means, for example, constantly adding the preparation every 2 weeks to control planarians. It is also important to note that after the 3-day course, it is necessary to siphon the soil to clean it from parasite remains and to change the water (so that snails do not get sick). The main active ingredient is Catechu palm seed extract rather than chemical elements, i.e., a natural material.

Sera Snailpur. With Sera Snailpur you will quickly and reliably get rid of the torment with snails, as well as freshwater polyps (Hydra), flatworms, and other parasites. Remedy against snails, hydras, and slugs in freshwater aquariums. Three days in a row, daily administer 1 drop of the preparation per liter of water or 1 ml per 25 liters of aquarium water. On the third day, replace half of the water. The filter should be washed. Treatment should be repeated in a week and two weeks to remove juveniles hatched from eggs during this time (egg clutches remain undamaged during treatment with this remedy). It is very important to monitor aquarium fish closely during treatment. Rapid breathing and gulping atmospheric water are symptoms of overdose. In this case, it is necessary to change the water immediately. Before placing new plants in the aquarium, disinfect them in a separate container for two hours using a double dose of Sera Snailpur. Then the plants should be thoroughly rinsed with water. Thus, you will reliably prevent the introduction of parasites. Shake before use!

Sera Snail-Ex. Remedy for combating snails, hydras, and flatworms in a freshwater aquarium. Before applying the preparation, you must turn off the filter and ensure good aeration in the aquarium. The pH level in the aquarium water should be above 7. Do not feed fish during the application of the preparation. To destroy hydra, add the preparation directly to the aquarium water for three days daily at the rate of: 1 drop per 1 l of water or 1 ml per 25 l of water. On the third day, change 50% of the water in the aquarium. Many species of catfishes and other bottom-dwelling fish, as well as black and rosy barbs, cannot tolerate the preparation, so the above-mentioned species must be transplanted to a separate container during application. During the application of the preparation, it is necessary to constantly monitor the behavior of the fish. In case of overdose, fish breathe rapidly and stay near the water surface, periodically gulping air. In this case, you should immediately make a 50% water change. To protect the habitat in your aquarium from introduced parasites, before placing new plants in the aquarium, you must disinfect them in a separate container using a double dose of Sera Snail-Ex for two hours. Then rinse thoroughly and place them in the aquarium. Shake before use!

ZMF Limnacid — A remedy for active control of snails in freshwater aquariums: acts quickly and reliably against snails, planarians, leeches, and other unwanted animals; does not harm fish and plants. Dosage: Against snails – 1 drop per 1 liter of water (one full screw cap holds 3 ml of the preparation, enough for 60 liters of water). Against hydras, planarians, and leeches – 2 drops per 1 liter of water (3 ml for 30 liters of water). Method of application: Before using the preparation, 1/3 of the aquarium water must be replaced with fresh water. Ventilate the aquarium and room well during the application of the preparation. During the application period, water must not be filtered through activated carbon and other adsorbing substances (ion-exchange resins), and fish treatments or water enrichment products must not be used.

JBL Schnecktol. Preparation for combating snails, leeches, planarians, and hydras, 100 ml. • Along with JBL Schneckenfalle, this preparation simply and quickly solves problems caused by snails. • After finishing treatment with the preparation, remove dead snails and those showing no signs of life from the aquarium. This preparation has no effect on trumpet snails. Dosage: 25 ml per 100 l of aquarium water; 1 outer hopper of the screw cap per 5 l of water; 1 inner hopper – for 30 l of water. Partial (about 20%) water change every two weeks.

Other aquatic insects, dangerous pests.

Along with food, other equally dangerous fish pests can be introduced into the aquarium. Dangerous pests that destroy fish include such aquatic insects as backswimmers, water scorpions (Ranatra), saucer bugs, water mites, tadpole shrimps, water striders, the creeping water bug (Aphelocheirus), as well as dragonfly larvae, water scorpions, great diving beetles, water scavenger beetles, and frolic beetles. All of them are visible to the naked eye, so with a certain amount of caution, sorting and carefully inspecting live food, you can avoid pests entering the aquarium. Some of them, such as diving beetle larvae and dragonfly naiads, suitable as food for adult cichlids and other large predatory fish, are completely unacceptable in an aquarium with small species and, moreover, fry. A single dragonfly larva, unnoticed by the aquarist, can destroy all fry and kill adult fish, such as neon tetras. Of course, these interesting, but dangerous creatures for fish must be removed immediately.

Insects (Insecta). It can be said that all representatives of the insect class are intolerable in aquariums where fish are kept. Many insects are predators, destroying both young and adult, sometimes quite large fish. Methods of combating them are very simple and limited to sorting food or catching animals that have entered the aquarium with a net. However, there are cases when small insect larvae, going unnoticed in a densely planted aquarium, cause irreparable harm. Great harm to fish in the aquarium can be caused by diving beetles and their larvae, as well as dragonfly larvae. Both are capable of destroying quite large fish. Young diving beetle larvae are especially dangerous because, due to their small size, they can be introduced into the aquarium along with bloodworms or crustaceans and remain unnoticed in it. Among water bugs, backswimmers are especially dangerous, whose strong poison injected into a wound during a sting is capable of killing quite large fish. Corixids can also cause great harm. Saucer bugs, water scorpions, and ranatras are also undesirable in the aquarium.

Great diving beetle (Dytiscus marginalis). To lay eggs, the female drills into aquatic plants with her ovipositor: water plantain, reed, and some others. Larvae hatch from the eggs and take six weeks to transform into adult beetles. During this time, young diving beetles molt three times. Both the beetle and the larvae need oxygen to live underwater. Therefore, beetles can be seen again and again at the water surface taking a supply of air under the elytra and then diving back into the water. Diving beetle larvae must also periodically surface to fill the tracheae-spiracles in their bodies with air through a tube at the end of the abdomen.

The hind legs of the diving beetle are covered with rows of brushes and are thus converted into swimming limbs. With their help, the beetle moves underwater. It can also fly. The head of the diving beetle larva is equipped with sickle-shaped jaws. With them, it seizes prey, injects paralyzing fluid through channels, then regurgitates enzymes from the stomach into the victim’s wound that dissolve tissues to a liquid state, and sucks out this liquid until practically nothing is left of the prey. The diving beetles themselves tear prey into pieces and eat it greedily. Cases are described where in natural bodies of water and spawning ponds, diving beetles and their larvae destroyed almost all fish fry.

Diving beetle larvae are small at an early stage of development and therefore may go unnoticed by the aquarist and be introduced into the aquarium with bloodworms, crustaceans, or other live food. With a certain amount of caution, sorting and carefully examining live food, you can avoid diving beetle larvae entering the aquarium. A cruel scene will also play out in an aquarium where you unknowingly put a few diving beetle larvae – they will start hunting each other. Such hunting will continue until a single larva remains in the aquarium.

Bryozoans are colonial sedentary (some can slowly crawl) animals, outwardly resembling mosses (hence the name). They reproduce sexually and by budding. They feed on infusoria and other planktonic organisms, which they filter out using tentacles. At minimum magnification, dense whorls of tiny tentacles become visible, serving to capture food, which is sent to a simple stomach. Several species of bryozoans inhabit local freshwater puddles, one of which is even “wandering” (a colony in the form of a flexible cake slowly moves across hard underwater surfaces). Most often, bryozoans appear from spring to autumn, at an aquarium temperature of 18-23°C at a shallow depth in fairly clean, established (but not “old”) water.

Occasionally, an “ice pattern” growing beautifully over several weeks, ranging from a few millimeters to centimeters in size, can be found on the aquarium glass – these are bryozoans. The presence of bryozoans in the aquarium does not harm its inhabitants. Due to their harmlessness and filter-feeding type, bryozoans are certainly a useful aquarium resident with adult fish and perhaps not so desirable when “raising” larvae from eggs due to food competition. To console the inquisitive, I will say that if you feed your aquarium animals bloodworms even occasionally, bryozoans surely live with you. Another matter is that they are not always so easy to discover.

Crustaceans (Crustacea). One of the most dangerous enemies of aquarium fish is the fish louse (Argulus foliaceus). This crustacean reaches 4–6 mm in length, its leaf-shaped body is semi-transparent, colored greenish-yellow, which makes the parasite poorly visible when sitting on a fish. Fish lice are very mobile, easily detach from the fish, and swim freely in the water using swimming legs. Areas of fish skin affected by the fish louse usually become inflamed; in more severe cases, parasites can cause deep wounds on the fish’s body. In addition, by destroying the skin, fish lice thus facilitate the penetration of microorganisms into the fish’s body and the development of secondary diseases. Combating fish lice is very difficult; in fact, it is limited to mechanically separating the parasite from the host.

Among dangerous crustaceans, besides fish lice, cyclops should be mentioned, as among them there are species capable of eating fry. Larger fry, however, eat these exact same cyclops without harm to themselves.

Seed shrimp Ostracoda are bean-shaped crustaceans reaching 4 mm in length. When catching fish food in close proximity to the bottom, typical bottom inhabitants of various fresh bodies of water – seed shrimp – can often be introduced into the aquarium. The bodies of these crustaceans are covered with a transparent bivalve shell. Sometimes they can be seen bustling about on the substrate like tiny moving specks. These creatures are yellowish or black-brown in color. They attach their eggs to plants, so they can be accidentally introduced into the aquarium along with plants, as well as live foods.

They are found in small quantities in aquariums, but if aquarium hygiene leaves much to be desired, they can begin to multiply rapidly and become a real disaster. Most fish do not eat these crustaceans. And although they do not cause visible harm in the aquarium, their presence is still undesirable because, by consuming oxygen, seed shrimp thus harm the main aquarium population. Thus, although seed shrimp are harmless, their presence indicates problems with the environment or fish nutrition. Improving aquarium care is a simultaneous solution to both problems. It allows controlling the number of these animals and eliminating the causes of their rapid reproduction.

Snails. Some aquarists intentionally introduce aquatic snails into the aquarium to serve as “cleaners” and clean up leftover food. Sometimes snails enter the aquarium accidentally – usually on plants. But no matter how snails got into the aquarium, if they turn out to be undesirable later, removing them completely is not easy. This is especially true for the live-bearing snail called Malaysian trumpet snail Melania tuberculata. These snails live in the substrate, where they can reproduce very intensively, and in such a way that the aquarist will have no idea they are there.

The presence of a large number of snails is an indicator of an unhealthy aquarium. For snails to live, they must not only find enough organic matter for food, but also consume oxygen and produce organic waste. Some of them devour fish eggs. Snails caught in the wild or in ornamental ponds can introduce many parasites into the aquarium for which they are intermediate hosts. The snail population can be kept within reasonable limits. To avoid accidentally introducing snails, it is quite acceptable to treat plants with a molluscicide before planting them in the aquarium. Remember that tiny snails may go unnoticed during a visual inspection of plants.

Annelids (Annelida) – various types of leeches. The coloration of horse leeches is olive-green, dark, sometimes almost black, the belly is lighter than the back; the body is segmented with two suckers at the front and rear ends. The large horse leech reaches 10–15 cm in length, the small one 4–6 cm. Large and small horse leeches are distinguished by voracity. The fish leech (Piscicola geometra) reaches 5 cm in length, 2-3 cm in width. The cylindrical body of this leech tapers somewhat towards the front end; the front, round sucker is about twice as wide as the body in diameter. The fish leech attacks fish, attaching itself to their skin.

Glossiphoniid leeches (Glossiphonia complanata) are distinguished by their small size (up to 2-3 cm), flat, wide, and transparent body of a yellowish or olive-brown color with numerous specks and yellow spots. Glossiphoniids stay on various underwater plants and objects (they cannot swim), so they rarely get into aquariums. Glossiphoniids attack mollusks, which is why they got their name.

In aquarium conditions, they are among the enemies that destroy fish and mollusks. Leeches swim using wave-like movements of the whole body in a vertical plane. In the aquarium, they can live in the substrate layer or on its surface. Their presence in significant numbers is an indicator of poor aquarium hygiene.