the red slime remover will remove it. It will come back if you haven't eliminated the source. Use it and continue the new feeding and wc routine and hope for the best. in the short term you can use a small amount of gfo. Also, If not already mentioned, water movement can really help. Look at that as a possible contributing factor. Good luck. These things ae a pita when they creep up on you and are really hard to turn around. It will happen though. Nothing good happens fast as they say.