I do normally clean the prefilter a lot more often than main filter compartment but maybe not weekly as i should.
Should be done weekly to stop the need for cleaning mail filter as often and to help with any reduction in flow hence helping to combat alge
None of the oase pre filter sponges need cuttingAre them pre-filter sponges correct size or need cut to size and i think may do same thing and replace the prefilters. But is that the logic that staghorn alage may be trapped in the prefilters or bad bacteria/alage is in the dirty prefilters?
If your pre filter sponges are dirty there going to reduce flow and not filter the water as it should be there fore giving alge the edge in the tank hence why your getting a little now I would think I would clean your filter out and start to clean the sponges weekly the pre filter sponges are not for any kind biological filtration