The reason you are having "scum" on the surface of the water is because you don't have a overflow which would pull water from the top getting rid of the scum. A more powerfull PH aimed at the top breaking up the water tension would help simply by mixing the scum back into the water. In the past when I've had a drilled tank without an overflow I simply used a PVC elbow with the opening at the water surface, it acts like asmall overflow. You could try to mod your plumbing by cutting your intake pipe and adding an elbow. If it doesn't work just get a joiner and reinstall your original pipe, you wouldn't even need glue as its underwater anyway. if you try the elbow be sure to get a joiner to go from your size PVC to at least a size or 2 up to a bigger elbow.
As for the build up on/ between your rocks just buy a simple turkey baster from wal mart or the supermarket and "blow" your rocks. I have a 900g seaswirl return and 2 tunze pumps in my 75g and I blow my rocks daily and you wouldn't believe all the stuff that gets stirred up daily when I do it.The reason is simple, no amount of flow can get everywhere in a tank, there is always a crevice or spot where one rock shades another and thats where things will settle. A 2$ turkey baster will take care of it without adding heat to the tank or ugly PH's .
One last thing, if you try the elbow, be sure to cover it with a piece of plastic gutter guard from Home Depo so the Sh's don't take an eventfull trip to the sump. do you have a sponge or something covering your intake now?