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Water Change Frequency
In a nutshell, the more frequent the changes, the less water that needs to be replaced. However, the longer between changes, the more stressful changes potentially become, 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 may not be enough. The proper frequency really depends on such factors as the fish load in your tank.
None the less, you should do water changes often enough so that:
- Nitrate levels stay at or below 50ppm, and preferably MUCH lower (less than 10ppm is a good optimal value).
- 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 test kit 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.
Water changes remove nitrates after they’ve been produced. Nitrogenous substances in the form of uneaten fish food, detritus, etc. can also be removed BEFORE they get broken down into nitrate. This is achieved by cleaning your mechanical and biological filter regularly, and by vacuuming the gravel with a gravel cleaner. This should be done every time you perform a water change, e.g., every two weeks.
Note: if your heater becomes partially exposed to air as the water level drops while doing changes, be sure to unplug your heater while doing your water changes. The heater can crack if the water level drops below the heating coil!
Also, be sure to dechlorinate/dechlorimate the replacement water before adding it to your tank!
Source: users.cs.duke.edu/~narten/faq/chemistry.html