Water quality drifts long before your fish show obvious stress signs. Testing on a schedule catches problems while they’re still easy to fix.
For most established tanks, test ammonia and nitrite weekly, and nitrate + pH every two weeks. New tanks that are still cycling should be tested every one to two days until ammonia and nitrite both read zero consistently.
Keep a simple log — date, each reading, and anything you changed that week (new fish, new food, a bigger water change). Patterns show up fast once you have a few weeks of numbers side by side.
If ammonia or nitrite ever reads above zero in an established tank, do a 25-50% water change immediately and re-test the next day rather than waiting for your regular schedule.