If it is so bad that you could drain the tank by siphoning it out with airline tubing then I would dose red slime remover and do a water change. The Gfo
and carbon would benefit your tank, mostly the Gfo
to remove phosphates which might be what is feeding this issue. Either way this is still a nutrient problem, either from the water coming in or it is being put in by over feeding. I just started an algae battle from me feeding too much again, using Julian's Thing instead of broadcast feeding has helped along with cutting back from daily feedings again.