A pic would help. Test for PO4
in your tank and the ro
water that u use to top off. If the resin is old or the PO4
level is very high in your tap water it may be passing through the filter. You should siphon as much as you can out when you do a WC
, I did and it helped/made the tank look better. It took me several water changes and using a phosban reactor to rid my tank of cyno. There are no critter that I know of that eat cyno, I tried every critter that they say eats it, crabs, snails and nothing touched it. Only through getting all new filters for my ro
and running phosban did I get rid of it. Good luck... First step, testing tank and top off water..