If you don't know the very basics of maintaining your tank, or if you take everything out and wash it about once a month, please read Basic Fishkeeping. (http://www.petfish.net/kb/6/) Generally, the more water you change per month, the better. I generally do 10% twice a week, which is really quite a bit. There are 4 things to consider in water changes: NitrAtes, pheromones, acid (All natural fish wastes) and fish stress. NitrAtes and Pheromones both cause stress, and can be removed from water though water changes. The changes themselves, however, also cause stress. Generally, small water changes are less stressful. Ideally, you'd do about 1% a day, although this is usually not practical. (it's also less efficient than larger changes) Try to do them frequently, and try to change about 40% over the course of a month. It's not usually a good idea to change over 30% of the water at a time, except in special circumstances like bettas in a bowl. Try also to make the changes gently: Don't let the tanks run while low on water for too long, and pour the new water in slowly. Make sure it's roughly the same temperature as the tank water. Remember to unplug any heaters you have, and immediately unplug any filter that begins sucking air. Acid will slowly build up in a fish tank until it overcomes the kH buffer. Water changes replenish the kH buffer. This is not usually a consideration unless you have very soft water. Also remember to rinse your filter media about twice a month in removed tank water (not tap water) and to clean your impeller and intake assembly about once a month. (if the impeller gets loud, it's gotten dirty)
