[djnzlab1]

HI,
I have had ich in the past, its usally a problem when the temp fluctuates to much or the carbonate is used up,
Do you have any crushed coral in the gravel , many times the carbonate buffer is used up by bacteria in the filters and the gravel, any PH swing is very hard on fish and cause NH3 spike if it drops to fast and there's alot of waste in the water and the filter.
My OP 95 octagon has a old saltwater sump with a filter pad most large fish mum gets in the filter pads so rinsing the pad weekly help clear the water and remove and free floating mum.
Another trick is to shut down all the filters while feeding.
many fish gill rake the food and there's alot of that food dust in the water column. so I use things like giant diano's to eat whast to small for the 6 and 7 in africans.
I have a red fin PLeco for the last 3 years he's fat on the waste from the feeding.
Another trick is to feed food that mostly biologically freindly to the water column, many food are to rich things like cichlid food will foul a filter in about 3 days. I have used that floating Koi food for cichlids primary diet for the last 20 years its very filter freindly floats so they can find it ( remember to pause the pumps I use a power strip for this). and feed what they need no more.
If your feeding a high protein diet many large fish will foul the undergravels to the point they are useless, and any pads will need frequent rinsing to filter.
Finally , i have a small marine canister filter for carbon, large fish pee alot and thats the best way to keep the water clear. and when the carbon is used up it becomes a good place for bacteria to breed in the canisters so I don't change it but every 6 mos or so or when the water become a little yellow
Never add extra salt to a system with carbon it can relase the organic trapped there.
Doug

Sorry for the longpost But I wanted to be sure you had all my tried and true facts.