How Often to Clean a Filter, and How Not To
The commonest way to crash a cycled tank is an enthusiastic filter clean.
A sensible interval
Rinse mechanical media when flow noticeably drops — for most tanks that is every three to six weeks. Biological media wants leaving alone far longer.
Use tank water
Chlorinated tap water kills the bacteria colony you have spent weeks growing. Rinse in a bucket of water taken out during the water change.
Never do everything at once
If you must replace biological media, replace part of it and leave the rest in place for a month so the colony can re-establish.