^^^This^^its not really about when to do a water change, but why to do a water change. Fish require water with certain parameters to live, ideally 0 ammonia, 0 nitrite, and low nitrate. When nitrate climbs high you perform a water change. Or when phosphates climb high you perform a water change to export the nutrients. Or if you have corals that suck elements from the water you perform water changes to replenish the elements. I perform weekly water changes, some people do it biweekly, some do it monthly, some very rarely. Everyones tank is different. I would start by testing your water and listing your parameters so that people here can offer some suggestions.
Sorry ph is 8.1Hi it’s a 120 litre tank running a fluval u3 and a protein skimmer no Corals just clownfish x2 hermit crabs x3 snails x3 and my nitrite levels have spiked so doing a water change today ammonia is good nitrite and nitrate are high ph is 7.9
thank youIf it is not cycled... then do water changes to keep ammonia low. Also dose some bio-spira to seed the tank. Ammonia is very toxic to fish, so that is the main thing to monitor.
Don't be shocked when everything gets covered in brown nasty diatoms.