Water Scum

[Wilty]

I'm new to the website, and been doing a lot of reading to catch up to where everyone is at. But I have a problem, well I think its a problem.

I get a scum across the top of the water (similair to an oil slick). Its been getting worse with the summer warmth and I didn't have this problem in the spring. I can suck it off the top of the water when doing water changes but it returns within a day. Water testing shows nothing wrong, and everything in the tank is happy. My protein skimmer is working fine and is skimming the water. Anyone seen this? I am not having algae problems. I found the "black powder" I'm using is having trouble 'raining down' on my tank since it get caught in the slick.

Tank specs.
33 gal (open top tank)
seaclone protein skimmer
fluval 404
60lbs of LR