As soon as aquatic plants and fish are settled in the tank your aim is to maintain stable conditions inside. For normal development of organisms and prevention of diseases the tank water should have a certain chemical compound.
The water is usually added to top as it evaporates, or while walls and substrate are cleaned. The total change of old water is needed very rarely. Usually this procedure affects plants: they fade and some parts die off. But even a partial water change may endure the gas and saline composition to drop. Therefore fishkeepers avoid total replacing even when a mass fish death happens. The total replacement of water is needed in three cases: 1) when harmful bacteria or mycotic mucous are found in the tank; 2) when water starts to blossom and darkening of the tank doesn't help; 3) when the substrate is too dirty.
In most cases regular topping up of water is desirable. But this way not more than 1/5 to 1/3 of the water volume should be changed.
When the water is replaced you must be absolutely sure it answers all the chemical norms for the fish occupying your aquarium.
The opinion quite popular among the beginners that the water must be changed frequently is absolutely wrong. This can lead to bad health and even death of fish. As a rule when the tank is correctly set (from the biological point of view), then the plants, the fish and the useful bacteria, present in the substrate and water, work as a good filter, and the natural balance may last for 10 years or so.
What is necessary?
Of course replacing 1/5 of the tank water with the tap water somewhat influences the balance in the aquarium, but within the day or two it will reestablish. The bigger is the tank, the more ability it possesses to keep the balance despite the clumsy interference of man.
One half of water replacement may lead to such a disbalance that some fish and plants may die, but in a week or two the homeostasis of the ambient will get to norm again.
Total replacement of tank water may completely ruin the environment and you will have to start everything over again.
* If you decided to be an aquarist but never have dealt with the tank before and wish to set everything as it becomes, try a small tank first (100-200 l). The natural balance and live ambient establish there as easy as in a tiny aquarium (20-30 l), but there is a lesser chance to destroy them.
The point is that we do not keep "aquatic plants and aquatic animals" but a "water environment". So the main task of an aquarium hobbyist is to maintain a well-balanced and healthy state of it rather than of its individual inhabitants, since healthy environment means good to all its live components.
During the period of an ambient formation (begins from the moment when the first plants are put and a week later the first portion of fish is placed) it is very unstable. Therefore it is strongly recommended not to interfere in this process. But what IS recommended then?
Do not replace water during the first two months. Adding a new sterile tap water to a "semi-tap" water (that has already begun to turn into a living liquid), makes no sense. In the big tank the water replacement hampers the establishment of the normal environment and in the smaller tank the interference will crash it down.
After two or three months the developing tank ambient reaches the stage of youth. From this moment and till the complete rearrangement of the aquarium you should once in 10-15 days (or once a month) replace 1/5 of water. The inhabitants do not seem to have a necessity for such a refreshing of ambient, but the ambience needs it to ensure a long youth and healthy maturity. While changing the water you may also clean the tank: gather dirt from the substrate with the rubber hose and clean the walls. Mind that the amount of water removed while cleaning should not exceed 1/5 of tank volume each month. For the tank larger than 200 l do the topping up by pouring a small stream of tap water inside. For smaller tanks the tap water is bad. Before topping up let it stand for a while till chlorine goes out, or heat it to 104-122 ?F.
Within half a year the environment enters the stage of maturity. From now only the clumsy interference may break the existing biological balance in the tank.
A year later it comes the time to help the ambient not to grow old. Clean the organic matter that has accumulated between the particles of substrate. Actually, once-a-month surface cleaning is enough to keep the substrate in a normal state even in a very big tank and to successfully restore the ambient. Thus for many years the tank will be save from a complete rearrangement.
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).