Perhaps the single most impactful task you can perform as part of your tank maintenance routine is to change your tank water...and do it often. Hobbyist fish tanks are typically closed systems. The water might be circulated through filters but it's still the same water. No old water is removed and no new water enters. So why take out some water and replace it with fresh? Among other things, changing the water does the following:
In addition, regular water changes can promote spawning behavior, especially in tough to breed species. Anecdotally, I've had pairs that seem to breed more frequently as the frequency of water changes increases.
If you visit online aquarium forums and boards, aquarists regular espouse water changes to solve all kinds of problems. There is a reason. Regular water changes will play as a large a role in maintaining a healthy tank as nearly anything else you can do.
How often should you change the water? How much water should you change? There are no single answers to those two question because there are many variables that factor in (tank size, tank filtration, tank inhabitants, source water). Performing water changes weekly is a good starting point. Changing a small volume (as a percentage of the total volume in the tank) will have little positive impact. Changing a large volume may have an enormous negative impact. Finding the right balance is where experience comes in. If you're a new aquarist, begin with ~20% each week. Test your water several hours after each change and see what your parameters are. A cycled tank should register no ammonia or nitrites, but it may take trial and error with the change volume and frequency to get your nitrates down to acceptable levels. The important thing to remember is simply to perform water changes regularly.
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