Feeding methods in fish.
There are lively fish that manage to gather food everywhere. For example, almost all livebearing toothcarps (Poeciliidae), whose feeding is therefore considered a simple matter. However, most species do not possess such skills. Thus, fish feeding from the water surface practically do not eat food that has sunk to the bottom; often their bodies are not even adapted for this. For instance, *Nannostomus* swims head-up, and *Dermogenys* never dive deeper than 10 cm.
On the contrary, bottom-dwelling fish cannot collect food floating on the water surface. Some of them manage to turn belly-up, but if the mouth is too small and they are used to nibbling rather than swallowing whole, such food is not for these fish. This remark applies primarily to small, narrow-bodied armored catfishes. Head-down swimming *Chilodus* and *Abramites* generally have such a mouth structure that they can only collect food in this position. *Anostomus* have adapted to graze algae from vertical planes, so to take food from the bottom, they awkwardly lie on their side.
Such features must be taken into account, otherwise, the fish will remain hungry. For example, spiny eels (Mastacembelidae) forage for their food by amusingly digging in the sand, while sniffing and probing for worms with their flexible and mobile snout. Often in search of food, they plunge their heads into the substrate and make sharp body movements in attempts to burrow deeper. Grains of sand scattering in all directions attract other aquarium inhabitants, which pick up the worms excavated by the spiny eels from under the substrate, while the eel fishes for new ones in the sand mass.
There is a method of feeding surface-swimming fish with tubifex or bloodworms, which tend to sink to the bottom. To do this, place the food in a non-sinking feeding ring, through the grid of which the worms will fall out, and the fish will immediately pick them up. If representatives of different species live in the aquarium, the feeder can turn from a helper into a problem. The fact is that different fish eat at different speeds, and if the shy discus takes a worm into its mouth, spits it out a few times and picks it up again as if tasting the food, during this time the gluttonous molly will already swallow a dozen worms, stubbornly pulling them out of the feeder.
Since the most popular aquarium species have a good appetite, an attempt to introduce timid discus, armored catfishes, or elephantfish to them will end in starvation for the latter. A small number of fast-eating fish can live in a large company of slow-eating species, but this is also not very good, as the former will always be overfed.
Most species search for food throughout the daylight hours, while others feed at twilight and at night. Fish obtain food in different water layers, so some fish take food mainly from the surface, most feed in the middle layers, and quite a few fish collect food from the bottom or search for it by rummaging in the substrate. Throughout a fish’s life, preferred food types may change. The size and types of food may depend on the size of the fish (in particular, the size of its mouth) and its stage of development. There are quite omnivorous species that eat both live and dry foods, while others are difficult to train to dry food. Observe the fish during feeding; at this moment, it is easiest to distinguish sick fish from healthy ones.
Fish differ from each other not only in preferring a certain diet (i.e., a specific type of food) but also in their feeding behavior. Many fish are predators, obtaining their food through various techniques. Predatory fish usually set an ambush, stealthily approach their prey, or freeze motionlessly, waiting for a suitable victim to swim by. Other predators, which usually chase their prey, rely mainly on their speed, trying to overtake and seize the victim, and having satiated, do not resume hunting for many hours or even days.
Fish usually determine the location of food using a system of organs that capture visual, electrical, mechanical (vibrational and acoustic) or chemical signals. By the physique of a fish, one can judge in which water region it prefers to live. This allows us to conclude which fish can populate the aquarium and how they should be fed. Depending on the feeding method, individual species during evolution have acquired a body shape and feeding apparatus that facilitate obtaining and consuming food. Fish taking food from the surface will ignore food falling to the bottom of the aquarium, and bottom fish will swim past floating food. The size, shape, and position of the mouth largely determine where and how the fish will feed.
Thus, there are fish with an elongated body, a straight back line, and a convex belly that stay near the water surface and even jump out of the water. The mouth of these fish is slanted upward (superior), which allows them, by maneuvering, to instantly snap up an insect that has landed on the water. They usually look for food on the water surface. They can be fed flake food, but at the same time they readily eat frozen dried larvae, insects, and sometimes flies.
Fish that inhabit the middle layers of water have a more or less evenly convex back and belly line. The mouth is almost strictly horizontal (terminal). These fish prefer everything that can be found in the middle layers of water, whether it is falling flake, live, or frozen food.
At the very bottom live fish that have an approximately rectilinear belly facilitating movement along the substrate, and a more or less convex back, and generally possess an enhanced sense of smell. The mouth is directed downwards (inferior) and has barbels—sensitive tactile organs for detecting food, or it has evolved into a sucker mouth. Bottom-dwelling fish feed on what sinks to the bottom or dig up the substrate in search of food. Some bottom-feeding fish need sand, which makes up 10-20% of their diet. It is used in the digestive process—to “grind” coarse food. And many catfishes from the family *Loricariidae* need a piece of driftwood to chew on.
Aquariums today contain fish that scrape algae from stones, glass, and other objects. In these species, the mouth is equipped with a sucker and acts as a scraper. These fish look for food at the bottom, which is thrown to them in the form of food tablets. Bottom fish do not refuse plant food either.
Fish whose diet includes mainly algae (for example, *Mollinesia*) can spend many hours feeding on algae and other plant food; their lower lip is wide, transformed into a lamellar scraper, with which they carefully clean the algal coating off plants. Young fish need special nutrition as they are still growing and have different nutritional requirements than adult fish.
Between these two types of fish, there are, of course, transitional forms that are more oriented towards the top or the bottom. Truly surface or bottom fish very rarely leave their habitats. Fish taking food from the surface will ignore food falling to the bottom of the aquarium, and bottom fish will swim past floating food. Even predatory fish with large mouths can be selective about food, and many of them will not waste time catching fry. All this should be known in order to be able to create familiar feeding conditions for your pets in the aquarium that correspond to their biological characteristics.
Fish kept in aquariums, although they do not live in natural conditions, need the specific food for which the structure of their digestive system is designed. Actually, the manner of eating, so to speak, differs among various fish species, and accordingly, the structure of the mouth and digestive organs also differs. The digestive system of fish throughout their history of existence in the specific conditions of their living space has adapted to the food encountered there. This is confirmed by the length of the intestine, which is proportional to the digestibility of the food.
In strictly carnivorous fish, which live more or less at the expense of other fish or other aquatic organisms, the intestine is very short and the stomach is large, serving primarily for receiving large prey, capable of stretching and accommodating single portions of large food items slightly smaller than the predator itself. Since such food is easily digested, a long intestine is not required.
In omnivores, the intestine is already longer, since a significant share of the diet is plant food, the digestion of which requires a longer intestine; their stomach is poorly developed or absent altogether. And finally, herbivores have the longest intestine, consuming hard-to-digest plant food. This must be taken into account when choosing food and preparing it, otherwise disorders in the digestive tract of the fish may occur.
Both the feeding strategy and posture are predetermined by nature, and in the aquarium, they can only be partially changed. Different species feed at different times of the day. Although most fish feed during daylight hours, some prefer feeding at twilight, and others at night. In captivity, fish often change their habits, but some remain faithful to their favorite feeding time.
The size of the food must match the size of the fish. The nature of the consumed food leaves its mark on the structure and armament of the mouth. Often inexperienced aquarists do not attach importance to the size of the food, not realizing that fry and small-mouthed fish cannot swallow a large larva, worm, or *Daphnia*, and they remain hungry while the food dies and decomposes. At the same time, large fish become irritated at the sight of food that is too small for them. Large fish will starve if fed small food; small fish find large food “too tough to crack” and cannot swallow it. There are cases of juveniles dying after swallowing food that was too large for them and unable to get rid of it. For example, fry of gouramis (*Trichogaster lalius*) can die after swallowing large crustaceans, and betta fry after swallowing *Chaoborus* larvae.
