Regular partial water changes in the aquarium.
In the aquarium, we keep not aquatic animals and plants, but an aquatic habitat, and the main task of the aquarist is to maintain an equilibrium, healthy state of this very environment, rather than its individual inhabitants, because if the environment is healthy, the inhabitants of this environment will also do well. A living aquatic environment in an aquarium is a model of a water body with a constant cycle of substances and the ability to maintain this cycle in an equilibrium state for a long time. The larger the volume of the aquatic environment capable of maintaining homeostasis (equilibrium development of all processes) within glass walls, the stronger this homeostasis mechanism, the more resistant it is against unskilled human intervention.
Hence the rule: the larger the aquarium, the harder it is to disrupt the well-being and viability of the aquatic environment. A viable aquatic environment forms at an equal rate in aquariums of any size, but it is harder to ruin it in a larger body of water. This is why beginner hobbyists are recommended to create an underwater garden in aquariums with a capacity of 150 liters and above.
The only correct way to keep aquarium fish is by having fresh water in the aquarium. What is “old” water? Let’s take an aquarium whose water has not been changed for about four years. The water has a characteristic yellow color, it tastes salty and contains a lot of toxic substances formed as a result of chemical reactions of the decay of biological residues. Pour a drop of such water onto a glass slide and let it dry. Undoubtedly, the amount of solid substances contained in the water will unpleasant surprise you.
And how does the fish feel in such a situation? Just like a person living next to a heavy chemical plant. They are used to stinky air, to water that has a taste, color, and smell; their body has adapted. And the fish in such aquariums have adapted to stale water, high salt content, and decay products. But try putting a fish there that has been kept in normal conditions. It will die within two to four weeks. But the worst part is that fish from such an aquarium cannot be placed in an aquarium with fresh water. The bottom line: the fish in the aquarium with “old” water is a hostage of its reservoir.
Recently, many new species of fish have appeared in our aquariums, whose well-being largely depends on the content of harmful substances in the water. It has been found with sufficient certainty that for many well-known, long-time inhabitants of our aquariums, regular water replacement with fresh water is highly desirable, and for some of them (for example, Symphysodon aequifasciatus) simply necessary. It is also known which species of fish, in what volume, and how often need their “old” aquarium water replaced with fresh water.
These requirements are, of course, different for various fish species. But on average, you should regularly replace approximately 25% of the aquarium water volume every two weeks, and in overcrowded aquariums, 30% every week. With a partial water change, its positive properties are preserved, and the biological balance is not disturbed. The point of the usually recommended partial water change is that fresh water enters the aquarium regularly and in small quantities, and thus there is no sharp drop between it and the old water. This is feeding the plants with new macro- and microelements, meaning we take away the old (unused) ones and give fresh ones.
Currently, there are several filtration technologies that allow denitrification in the filter, yielding zero levels of ammonium, nitrates, and phosphates even at the highest fish stocking density, but they are applicable only to a “fish-only” aquarium without plants. Thanks to such technologies, water changes can be reduced to a minimum of 10% once every month or two.
An aquarium with plants needs a source of nitrogen – nitrates and ammonium; in case of slowed plant growth, the applied fertilizers should not accumulate indefinitely. A much higher lighting intensity and the presence of all nutrients for algae do not allow keeping a lot of dissolved, hard-to-decompose organics in the water, so water changes are indispensable. This also has a very beneficial effect on plants and the entire aquarium. Water changes activate plant photosynthesis.
One of the solutions to the pollution problem is dilution. Part of the dirty water is replaced with the same part of clean water, effectively diluting unwanted substances in the aquarium while simultaneously removing some of them. In addition, partial water replacement allows restoring the reserves of necessary mineral substances. Water changes are a way to add trace elements to a planted aquarium. In aquariums where part of the water is regularly replaced, the pH is usually above 7, and alkalinization is not required. Those aquarists who only top off the aquarium (to replace evaporated water) are doing it wrong, as such actions can lead to an excess of salts in the water and, consequently, to a disruption of its salt composition.
The living aquatic environment established in a spacious aquarium will correct minor mistakes of the underwater gardener and restore its homeostatic state. It cannot do this in three cases: • sharp intervention of the gardener in its existence — all maintenance of the underwater garden must be built on the principle of smoothness, maintaining stability; • too large a volume of replaced water — in this case, the environment is destroyed, resulting in a mixture of remnants of the living environment and lifeless tap water; • overfeeding the fish living in the underwater garden, accumulation of their food residues — instead of a living aquatic environment, you get an infusion of feed in which neither plants nor fish can exist.
Measure the distance from the bathroom (or maybe the kitchen) to the aquarium and buy a transparent hose with an inner diameter of 8-12 mm (with a margin). The hose for vacuuming the substrate should have a larger diameter than the gravel, or be transparent so that it is visible if 
a clog has occurred. The thinner the tube, the lower the water current. When buying a hose, pay attention to its material. Do not waste money on all sorts of corrugated, reinforced, and other heavy-duty structures intended for use in the country house. Aside from losing lightness, compactness, and elasticity, you will achieve nothing.
Excessively rigid hoses are inconvenient for regulating the water flow and can pop out of the aquarium. The most suitable option is hoses made of flexible polyvinyl chloride, i.e., the same type used for aquarium aeration. It is best to store it in a heated room, compactly rolled into a coil so as not to form kinks, where cracks or narrowings of the hose may eventually develop, leading to the formation of “blood clots” of aquarium debris removed along with the water change.
To prevent the hose from slipping off the drain spout and popping out of the aquarium, it is advisable to provide some brackets, clips, or clamps to ensure its secure fixation. You can take a piece of plastic pipe of such a thickness that the hose fits tightly into it (1-2 mm larger than the hose diameter) and bend it very carefully along a large arc so that the pipe does not break or squeeze the hose.
In the bathroom, we will need a drain, the entrance to the drain pipe. You can install or use the existing V-shaped corner for draining water from the washing machine under the washbasin. If impossible, you will have to drain the dirty water into the bathtub or toilet, securing the hose with a reliable mount and installing a washing machine drain adapter on the rim.
There is another device that greatly simplifies working with it. As a rule, with most siphons, the hose with the drained water is lowered into a bucket. And so that the end of the hose does not pop out, it has to be held with a free hand. But if the height of the aquarium is large, this is not always convenient. Sometimes you have to let go of the end of the hose and use your second hand to lift or move a decoration in the aquarium. An unpleasant surprise may occur. The hose will pop out of the bucket, and naturally, puddles on the floor are guaranteed. And this is where a mount is provided for this case. It is attached to the vertical wall of the bucket, and the hose is threaded through it. In such a secured position, the hose definitely will not pop out of the bucket while both our hands are busy with the decoration and substrate in the aquarium.
Generally, this operation is needed to reduce the amount of decay products and, most importantly, by changing the water, lower the content of final decay products – nitrates. Water changes remove excess NH4, PO4, NO3 and their source – organics, and also remove algae spores. At the same cases, the causes of imbalance are eliminated – fish overpopulation, overfeeding, silting of the filter or its insufficient performance; CO2 fluctuation causes are eliminated and its concentration is brought into correspondence with the lighting level, plant growth rates, and the phosphorus-nitrogen dosage is normalized. During the establishment of the nitrogen cycle, water changes also reduce ammonia and nitrite concentrations.
Water replacement is one of the main ways to combat algae (reducing phosphate concentration). Regular water changes are one of the effective ways to fight fish diseases. It is necessary to carry out frequent changes of large volumes of water during fish illness. If fish show symptoms of poisoning, a partial water change must also be carried out urgently. Similarly, during fish treatment, partial changes remove medicine from the aquarium once it has completed its main task. It should also be said that with each water change, various additives are also removed – for example, plant fertilizers, CO2, etc. The deficiency of these substances must be replenished with the appropriate dosage.
Often water in the aquarium is changed when it becomes cloudy. In a newly set up aquarium in the first days, the water may be cloudy due to the rapid reproduction of 
unicellular organisms in it. The habitat during its formation period (when plants are planted in the substrate, and a week later the first fish are introduced) is extremely unstable, so interfering with the aquarium’s operation at this time is strictly prohibited. The most dangerous is green cloudiness from the rapid development of green algae. For any problems with algae growth, the first thing done is to increase water changes up to twice a week by 30%.
Sometimes water becomes cloudy from a multitude of bacteria developing in it, which indicates that not everything is well in the aquarium. Most often this happens in the presence of a large amount of organic substances formed as a result of rotting, less often due to the accumulation of mineral salts. The water in the aquarium turns cloudy from the appearance of a large number of putrefactive bacteria, which are very harmful not only for fish but also for aquatic plants. The cause of such bacteria appearing is improper feeding and excessively dense stocking of fish in the aquarium.
In an overcrowded aquarium, water turbidity can occur if it is not sufficiently aerated or filtered. In such an aquarium, accumulating metabolic products serve as a good breeding ground for the mass reproduction of bacteria and unicellular organisms. Bacterial cloudiness is persistent; one must immediately start a serious fight against it, otherwise, mild turbidity will quickly turn into significant cloudiness, complicating the fight against it. Rapid bacterial reproduction can manifest in your aquarium every time a large amount of water is changed. Such turbidity, if its cause is eliminated and you wait for some time, disappears.
Large water change in the aquarium. Using a water change filter.
What to do if the aquarium water is cloudy or has an unusual color? 
