It could be rocks leeching crap... I have this problem now in my reef, after bleaching the rocks... GHA everywhere...
Astrea snails are helping a lot. Dosing NOPOX do nothing anymore with me, the levels are all undetectable, added GFO, dosing nopox...
This is the rocks wich leech something, so algae feed on the crap, and not the bacteria combined with NOPOX...
Any Pictures ?