Water changing frequency

The more frequent the changes, the less water that needs to be replaced. However, the longer between changes, the more stressful each change potentially becomes, because a larger portion of the water gets replaced. Replacing roughly 25% of your tank´s water bi-weekly is a good minimal starting point, but this may not be enough. The proper frequency really depends on such factors as the fish load in your tank, how much you feed at each feeding and what types of filtration you have. Nonetheless, you should do water changes often enough so that nitrate levels stay at or below 50ppm in fish only systems, and preferably MUCH lower (less than 10ppm is an optimal value) while reef tanks should have a nitrate level below 2 ppm; the change in water chemistry resulting from a change is small. In particular, the before and after pH of your tank shouldn´t differ by more than .2 units. Use a ph test kit or meter the first few times to get a feel for what´s right. If your pH changes too much as a result of a water change, perform changes more frequently, but replace less water at each change. Make sure the water temperature does not fluctuate more than 2 degrees when you perform a water change.