Jump to content

Water Change Guidelines


Recommended Posts

  • Regular Member

There is a lot of discussion and advice offered on how often water should be changed and how much Thought that I would add my own two cents based on calculated dilution effects for different fish loads. You can estimate fish load by testing for nitrate after you do a water change, and before you do the next change (say one week later). The increase in nitrate concentration is an indication of the fish load (assuming a cycled tank). This can be put in a neat table, but I can't seem to get it to format properly. So here goes.

If you measure an increase in 20 ppm nitrate over your typical water change interval (say one week), and you change 10% of the water over the same period, your nitrate concentration will stabilize (after multiple cycles) at 200 ppm. Assuming same fish load, but a 25% water change (after multiple cycles) will yield 80 ppm. 50% water change yields 40 ppm.

If you measure 10 ppm over your interval, 10% water changes yield 100 ppm, 25% yields 40 ppm, and 50% yields 20 ppm.

If you measure 40 ppm over your interval, 10% water changes yield 400 ppm, 25% yields 160 ppm and 50% yields 80 ppm.

It doesn't matter what your intiial concentration is. If you start higher than the predicted level, your concentration will decline to the predicted level. If you start lower, it will increase. The predicted level is the level after the water change.

If I've confused you, here is a specific example. You want to change your water once per week and can't decide how much to change. You measure your nitrate level and find it to be 60 ppm. You wait for one week, measure again, and find it to be 70 ppm. That's an increase of 10 ppm. Based on the information above, if you do a 25% water change every week, your nitrate level will stabilize at 40 ppm (after a few weeks, and will range over each week from 40 ppm to 50 ppm). If you think that this is too high, you can increase the amount you change to 50%, and your nitrate level will stabilize at 20 ppm. Etc.

Hope this gives you a better basis for deciding on how much and how often to change. This also shows the importance of fish loads, and why you shouldn't overload your tank (and the price you will pay in water changes if you do). Have fun.


Link to comment
Share on other sites


This topic is now archived and is closed to further replies.

  • Create New...