The solution to pollution is dilution; water changes replace a portion of “dirty” water with an equal portion of clean water, effectively diluting the concentrations of undesirable substances in your aquarium. In an established aquarium, nitrate is the primary toxin that builds up. Regular water changes are the cheapest, safest and most effective way of keeping nitrate concentrations at reasonable levels.
During the aquarium cycling phase, however, ammonia or nitrite may be the substances that need to be diluted and removed. Likewise, if medications have been added to your aquarium, they may need to be removed after they’ve served their primary purpose. The effectiveness of water changes is determined by two factors: their frequency and the percentage of water that is replaced.
The more often water is replaced, or the greater the quantity of replaced water at a change determines overall effectiveness. The benefits of water changes must be balanced by the stress caused by a sudden change of your aquarium’s water chemistry. If aquarium water has similar pH, GH and KH as tap water, changing 50% (or more) of the water at one time will not affect fish.
On the other hand, if your aquarium’s pH is (for example) 6.3, while your replacement water has a pH of 7.5, replacing 50% of the water all at once will change the pH of your aquarium significantly (possibly more than 50% depending on buffering factors), which will stress your fish, possibly enough to kill them. Because water changes are the first line of defense in dealing with problems such as disease, you want to be able to do large, frequent partial water changes during emergency periods. A good rule of thumb is changing 30% of your water tank once a week.