Welcome to 3reef!

I'll let the experts answer those questions, but I do have a question for you. How's the SeaClone working for ya? Are you getting gunk in the cup or just a bunch of water? IMO I'd get rid of that thing(I had one) and buy an AquaC Remora. Much better skimmer.
Also, you said a 55gal? I think you are WAY overloaded with fish... especially the two tangs, they need room to roam, and those Chromis' will grow too......
I'd have at least 2 Koralia 3's in there(powerheads) Do you have Nitrate or algae problems? How old is tank??? Let's see some pics!!!