How long have you had them? If they are a new addition, what lighting were they under in the previous system? Don't let your guard down, this could be the start of a meltdown. Do a pest dip, then a furan2 treatment. Not saying the lighting isn't the problem, but t5's are enough for zoanthids. I once kept zoanthids under a power bright l.e.d strip as their only light, they did fine and grew. My lfs did the same with gsp, and ricordea.