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 qantity 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 tank’s water chemistry. If tank 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 tank’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 tank 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.

Consequently, you want your tank’s water chemistry to closely match that of your replacement water. That way, you always have the option of performing large water changes on short notice. Note that this is the way tanks start out; when you initially set up your tank, the water is the same as that from your tap. Over time, however, the tank’s water chemistry may “drift” relative to tap water due to acidification from the nitrification cycle, the addition of chemical additives such as “Ph-up” or “Ph-down”, the use non-inert tank gravel (e.g. crushed coral or sea shells), etc.