I have a newish tank I understand that the filter must be cleaned in tank water so the ?balance is not upset How then do I clean the filter if I am not doing a water change? It is a cold water tank with minnows if that makes a difference
In my experience minnows are fairly resilient fish ( least ours are). We never do a complete change of water. Usually a bit at a time so whenever we empty some of the tank water out, we clean the filter out in the old tank water, so all the 'mank' comes out in the old and not the newer water. Hope this helps
Usually you would do water changes more often than the filter is cleaner- the water quality will dip before the filter clogs up. In my 60 litre tank I clean the filter every other water change (so once a fortnight) and in my 70 litre I do it once every 3 or 4 weeks; I only have 4 guppies in there so it takes ages to clog up.
It takes a bit of trial and error to find a cleaning regime that works for you because a lot depends on stocking level, how much food they have etc. The aim is to clear enough of the crap to let the water flow through. The good bacteria need a constant flow of water and if the sponge gets blocked, they start dying off. The sponge doesn't need to be 'clean'- it's fine if it still looks a bit grubby, as long as the holes aren't clogged up.
If there's carbon inserts in the filter then they need to be changed every 4 weeks- which is why I don't use them It works out pricey and it's not really necessary in your average properly stocked and maintained tank.
Are you testing the water to see how long it takes for the nitrAte level to rise? How big is the tank and how many minnows do you have?