Stubborn Protein Film

[reefcat]

So my tank has been going along fairly well lately, but one thing I can not seem to fix is a fairly thick protein film on the top of my 30g. I do 20% water changes every 2 weeks, my skimmer is skimming about a half a cup a week and i have one koralia nano powerhead running 9 hours a day along with the 205 canister. I know thats not a lot of flow but every time I try and add more, like another nano skimmer, or even a 750gph fish panick, all my corals (all lps) close up. Ive tried removing it manually with the net and it only goes away for a few hours. I would love to find a way to get rid of this..