Hello. It depends on the size of your tank, how many fish are in it, what types of fish you have, (for example gold fish are really poopy and dirty the water faster than many other types.) Also, if you tend to over feed, that can foul the water much sooner. Generally speaking, once every two weeks will be enough to make a partial change, changing out at least 10% of the total volume. But instead of trying to set a certain time schedule, it's best to keep an eye on the clarity of the water, and keep up with your ammonia, PH, nitrite and nitrate levels. You can do that by picking up a water testing kit at your local pet store, or if you have a store that has knowledgeable employees, you can take a sample in from your tank and they can test it for you. Most stores do that free of charge. They can also tell you what's right, what's wrong if anything, and what you need to do to correct it. I hope this helps but if you need anything else, just let us know. Patricia
Parrot C&onsultant
Published author, free lance bird behaviorist, adviser to the parrots at Sarasota Jungle Gardens.