Inhabitants of the saltwater aquarium.
When keeping fish and plants in aquariums, the question of water salinity often arises. Why do some need salt water, while others need fresh water? Why do many plants grow poorly in brackish water? How to change water salinity? The correct solution to these issues will determine the optimal conditions for the life of aquarium inhabitants, since metabolism between the organism and the environment is closely related to the chemical composition of water. Since much more salts and minerals are dissolved in salt water than in fresh water, the oxygen level in sea water is about half lower than in fresh water.
All natural waters are subdivided (in ppt/‰) into fresh up to 0.5, brackish 0.5-30, marine 30-40, and hypersaline over 40. Brackish, or mixohaline, waters, in turn, are divided into oligohaline 0.5-5, mesohaline 5-18, and polyhaline 18-30. An aquarium in which the water has a salinity of 5 to 10 ppt is called a brackish water aquarium. The paradox of brackish waters is that the minimum species zone, determined by the relative immiscibility of marine and freshwater faunas, does not fall on the middle of the interval (15 ppt), but on lower salinities of the order of 5-8 ppt. This is a kind of salt barrier separating typically freshwater and typically marine fauna: freshwater hydrobionts cannot tolerate an increase in salinity to 5-8 ppt, and marine hydrobionts cannot tolerate its decrease to such a level. At a specific gravity salinity of your aquarium water of 1.015, plants begin to die off.
The synthesis of substances, respiration processes, and decomposition of complex compounds take place in the cells of living organisms. In the process of life activity, some substances are constantly consumed and others are formed. Part of the newly formed molecules remains in the cell, part is transported to other cells or excreted into the environment. To ensure the life process, a constant supply of initial components and removal from the cell of by-products formed during biochemical reactions are necessary.
The content of various salts affects which substances and in what quantities will enter or leave the cell. The rate of penetration of ions through the membrane also depends on the difference in concentrations of a given ion on both sides of the membrane. The greater the concentration difference, the more ions diffuse towards their lower content. In addition to diffusion, which occurs due to the difference in concentrations, there is active transport of ions, in which movement is carried out due to the difference in electrochemical potentials through special areas of the membrane. This movement can also be carried out from a lower concentration to a higher one.
Simplified, the structure of a living cell looks as follows: inside the cell wall (a relatively rigid formation) is the protoplast (the living part of the cell), which encloses all cellular organisms contained in a complex solution – the cytoplasm. The cell wall has selective permeability for various ions, meaning different substances penetrate the membrane at different speeds. This is determined by their different solubility in individual membrane components and different pumping speeds during active transport. As a result, an uneven distribution of a number of substances on both sides of the membrane is formed. Plant cells actively pump potassium, while sodium, which is close to it, is conversely pushed out into the environment.
The set of processes regulating the osmotic pressure of body fluids is called osmoregulation. This process is found in most organisms. In freshwater fish, water along with the salts it contains actively enters the cells through the body surface and gills and is excreted from the body through the kidneys. In saltwater fish, water that enters the body is excreted through the skin, and NaCl is excreted mainly through the gills by special glands. Aquatic plants and freshwater fish satisfy the body’s need for ions by absorbing them directly from the water. If it does not contain the necessary elements, then with a normal ratio of osmotic pressures, a change in the content of individual elements occurs, that is, a change in the ratio of ions in the body. In a number of cases, this leads to a violation of biochemical processes.
When Na+ is absorbed, a large amount of energy is required. During the assimilation of Na+ by the organism, it is replaced by NH4+. Ammonium can be excreted by the organism from organic nitrogen-containing compounds. Thus, increased sodium consumption leads to impaired protein metabolism. In plants, an increase in sodium concentration leads to the blockage of potassium ion entry through cell membranes. The plant may experience potassium starvation even with a sufficiently high absolute potassium content.
Saltwater aquarium.
Where rivers flow into the sea, salt water mixes with fresh water. These areas are called river mouths, bays, estuaries, mangrove swamps, and salt marshes with low salinity of 3-5 ppt per liter. Many livebearing killifish, needlefish, some mollusks of the genus *Neritina*, gobies, and sleepers live in them. Water that is partially salty and partially fresh becomes hard. Salt water is heavier than fresh water due to the presence of many other salts. When they meet, the flowing fresh water stays on the surface.
From this, it follows that mixing does not happen instantly; low salinity is at the surface and very high at depth, and it takes some time for mixing. In addition, the continuous movement of tides and ebbs into the river changes the salinity and hardness of the water. Therefore, fish living in these conditions have developed over time a certain resistance to sudden changes in water salinity that occur daily.
These brackish waters are home to an amazing, diverse, and unique group of fish, some of which are available for home maintenance. Some fish are found in river mouths and mangrove swamps with slowly flowing water, while others are found in the upper salty waters of fast-flowing rivers. In most cases, the water in mangrove areas is muddy, with a thick layer of silt on the bottom. Most of the fish commonly found in brackish aquariums from these areas have adapted to migrate from the sea to rivers and vice versa.
Always check the compatibility of fish; some species from the same habitat cannot be neighbors in an aquarium. The low salinity of brackish water can have a specific gravity of 1.002 to 1.005. While measuring salt content by weight is great for estimating how much salt to buy and use, you definitely need to measure the specific gravity of the water.
In order to keep fish in conditions as close to natural as possible, they can be divided into 3 groups:
1 – Estuary (*Monodactylus*, *Scatophagus*, *Arius*, etc.). Lower mouths of rivers flowing into the sea. A large aquarium is desirable for them. The water current in such biotopes ranges from slow to moderate, 
therefore water movement from the filter must be adequate. Rocks, driftwood, or branched roots can be placed in the aquarium. Water changes in the aquarium are made up to 30% per week. In principle, such an aquarium functions on the freshwater principle. Very young fish are bought, and salt is added starting with 2 tablespoons of sea salt per 4 liters of water. Every 6 months, another tablespoon of salt per 4 liters of water is added during water changes. Stop when the addition reaches about 7 tablespoons of salt per 4 liters of water or when your hydrometer reads 1.020. This process will take about 3-4 years and should not happen too quickly. Keep in mind that you should not add salt directly to the aquarium.
2 – Brackish river (*Brachygobius*, *Stigmatogobius*, *Poecilia* (*Mollinesia*), *Chanda*, *Telmatherina ladigesi*, *Tetraodon nigroviridis*, *biocellatus*, etc.). Upper mouths of rivers flowing into the sea. Very 
often wetlands extend far beyond the riverbed. These are areas of rapid flow, and an additional pump must be installed to increase water movement. Aquarium from 20 to 100 liters. The design uses high and low driftwood, stones, and caves for gobies. Plants *Anubias* and *Microsorum* can be attached to rocks that form caves. The water is changed to brackish, and fresh water is not topped up. Due to evaporation, water salinity will increase. Add 1-2 tablespoons of sea salt per 4 liters of water. A water specific gravity of 1.005-1.010 will be sufficient for the full life of most of these fish.
3 – Mangrove swamps (*Toxotes*, *Periophthalmus*, *Anableps*, etc.) – mangrove zones with a salinity of 8-12 ppt per liter. 
Aquariums are filled 75% with water and 25% with land partially raised above the water. For this reason, use an aquarium of 150 liters or more so that there is enough swimming space for the fish. It is better to use an external filter for filtration. Plants will not grow in this biotope. Mangrove thickets, decorative roots, and plastic plants are installed along the water’s edge on the “mainland” side. About 3 tablespoons of sea salt per 4 liters of water with a specific gravity of 1.010 is enough for the fish to live a full life.
The aquarium must be covered with a lid glass; keep in mind that brackish water evaporates faster than fresh water. It is good to create one high focal point where stones are stacked rising at least to half the height of the aquarium, with open water on the sides. You can simply place three or four large flat stones on the bottom and then place one large slab on top of them, repeat this type of structure several times to create as many caves as necessary for the fish.
Plastic plants can be very useful, but you should only use two or “three” species in large quantities; a variety of species just looks like a pile-up. The shapes and colors of plants depend on lighting: if the aquarium has a dark substrate and background, then dark green and red plants are better, but if the aquarium has a light background due to limestone and sand, lighter plants are taken. Plants should be a complement to the design, not contrast with it.
The substrate is chosen from non-toxic coral sand or limestone and marble gravel. If you plan to grow plants, the substrate height is about 2.5 cm (anaerobic zones develop in higher layers), and if only for fish, it is better with a very thin layer of sand. Use driftwood and twisted roots to recreate the look of mangrove swamps. Empty snail shells can be used. Make sure the heater is rated for fresh and salt water use. You will need a thermometer and a hydrometer to measure water salinity.
The water needs to be hard and alkaline. The pH level must be maintained in the range of 7.5-8.0, and the hardness level from hard to very hard. Water density is about 1.005, values between 1.002 and 1.008 are quite acceptable. Filtration must be efficient because brackish water fish are heavy eaters and produce a lot of waste, and they are also sensitive to pollutants in the water. Be sure to change 15% of the aquarium water once a week.
Fresh water should be added within the same salt dosage as before to prevent osmotic shock in fish. It is better to use synthetic sea salt in the aquarium, which can be purchased at most pet stores. A less expensive substitute, but much poorer in quality, is regular table salt. Under no circumstances should you use iodized table salt; it can be very harmful to fish.
Once a week, it is necessary to mix the sand from the back of the aquarium to the front; this mixing will help prevent the formation of anaerobic spots. After mixing, remove debris from the water surface. It is advisable to feed the fish 2 times a day. Brackish water fish prefer quite warm water (at least 26°C), and this increases the metabolism of the fish, making them eat more food and grow faster. Many of these fish (*Mollinesia*, *Monodactylus*, *Scatophagus*, *Stigmatogobius*) require a certain amount of plant food in their diet, and pufferfish need snails. Temperature from 26 to 28°C, pH 7.5-8.5, dH 10-20°.
Material translated from http://badmanstropicalfish.com/brackish/brackish.html
Schooling fish in the aquarium.
Rules for keeping aquarium fish. 
