How often should you perform water changes?
Replacing 1/5 of the water (by adding tap water), within reasonable limits of course, will disrupt the equilibrium state of the environment, but it will recover within a day or two. Replacing half of the environment will disrupt the equilibrium stability, some fish and plants may die, but within a week or two, the homeostatic nature of the environment will recover again.
Many novice aquarium hobbyists prefer to change all the water in the aquarium every month – this must strictly not be done. Replacing all the water with tap water can completely destroy the environment, and everything will have to start over. The larger the aquarium, the greater its resistance against our unskilled interventions.
Regular water changes depend on the biological load on the ecosystem – the number of fish and the ability of plants to process their waste, i.e., the lighting level of the aquarium and the type of most plants – slow-growing or fast-growing. This boils down to the fact that in an aquarium with a low lighting level and slow-growing plants, water changes are carried out once a week, and in an aquarium with fast-growing plants and more intense lighting, changes are done every 2 weeks.
Water changes also depend on the condition of the aquarium – the presence of algae. At the slightest sign of algae, water changes are increased, and at the same time, the causes of their appearance are eliminated. Some people only top off the water. This is also wrong. Distilled water evaporates from the aquarium, while all trace elements and salts remain in your aquarium. This leads to an increase in general hardness (GH) and carbonate hardness (KH) of the water.
For two months in a new aquarium, water should not be changed – what is the point of introducing sterile tap water instead of semi-tap water that is just turning into living water? In a large aquarium, water replacement will slow down the formation of the habitat, and in a small one, this intervention will cause a catastrophe and everything will have to be started over. In two to three months, the developing aquatic habitat in the aquarium will enter its youth stage. From this moment and until the complete redesign of the aquarium, you should start replacing 1/5 of the water volume every 10-15 days, or even monthly.
The inhabitants of the aquarium do not seem to need such a renewal of the environment, but the habitat needs it to prolong its youth and maturity. During a water change, you can clean up – use a hose to collect debris from the substrate, and clean the glass. Water from the tap is added in a thin stream to an aquarium with a capacity of more than 200 liters. For small bodies of water, the water needs to be settled in the room. After half a year, the maturity of the habitat arrives.
Now only gross intervention can spoil the established biological balance in the aquarium. After a year, it is time to help the habitat not to age. It is necessary to remove accumulations of organics in the substrate, i.e., clean the substrate. Regularly washing the substrate, it is necessary to ensure that the total mass of debris removed with water does not exceed 1/5 of its volume in the aquarium; even in the largest home body of water, it is possible to gradually wash all the substrate. In return, we rejuvenate the habitat for a whole year, and a year later we repeat this operation again. In this way, degradation of the habitat is prevented, and the aquarium brings joy to its owner without major remodeling for many years.
Water changes are the only way to remove organic compounds. There is a group of compounds in water that are not destroyed by bacterial oxidation and are not removed from the aquarium either by natural self-cleaning or by biological filters. They can only be eliminated mechanically, namely: in nature – current, in the aquarium – only by partial water changes. The more frequently the water is replaced, the smaller the amount of water needed for such replacements.
However, the less frequently the water is replaced, the greater the potential stress level of the fish becomes due to the larger amount of water being replaced at once. Replacing about 25% of the water every two weeks is the minimum acceptable, but not sufficient norm. The necessary frequency of water replacement depends on many factors, including the number of fish and their requirements.
The frequency of water changes and the volume of water replaced can vary for different aquariums and depend on a number of factors – mainly on the density of fish and plant populations relative to the water volume, as well as on the amount and type of food offered to the fish. It is better to do this in small doses to avoid shock from sudden changes in water composition. Nevertheless, it is necessary to change water regularly and often enough so that the nitrate level in the water does not exceed 50 mg/l, and no significant changes in water parameters occur. In particular, before and after the change, the water pH level should not differ by more than 0.2 mg eq./liter. If changes exceed 0.2 mg eq./liter, it is necessary to change the water more frequently, while using a smaller amount of water at a time.
An aquarium, like any dwelling, needs regular cleaning. When cleaning an apartment, it is usually aired out immediately. When you change the water, it is useful to clean the bottom. By doing this, you make the work easier for the bacteria that decompose organics. It is best to siphon the bottom – while you siphon and clean the walls, a quarter of the aquarium drains out. Before you start siphoning water, it is worth “stirring up” the substrate a little so that the dirt inside it comes to the surface. During the cleaning process, you will replace a part of the water in the aquarium. There is nothing complicated about this procedure, especially since it is enough to perform it just once a week, and if there are few fish, twice a month is enough. After the aquarium is planted with plants and populated with fish, the hobbyist should strive to maintain a stable regime in it.
At one time, you can replace no more than one-third of the water (exceptions are critical cases, for example, serious poisoning). To begin with, it would be good to replace one-fifth to one-fourth of the water once a week while checking the nitrate level. The main mistake of beginners leading to fish death is the rapid replacement of a large volume of water. It is better to change water in smaller portions (10-20% of the aquarium volume) than rarely, but in large amounts. If you haven’t changed the water for a long time, you shouldn’t change a lot of water at once; start with 10%. This prevents drastic water parameter changes that can cause stress in fish.
The importance of aquarium water temperature. What temperature to keep fish at.
What water to pour during water changes. 
