oil slick

[roundman]

I have a 75 gal fowlr tank with a prizm pro skimmer, a fluval 304 that only has the sponges and small pieces of lr in it, and 77 lbs of live rock.I have the following 2 clowns, copperband, 6 line wrasse, 3 chromis, royal gramma, 12 blue hermits, 2 red hermits, serpent starfish, emerald crab, 10 snails.
There is a oily slick on the top of water is this normal and if not what can be done about it.
thanks

 

[scsinet]

It's likely a protein scum. Even with tanks with skimmers this happens if there is no surface skimmer as proteins just accumulate on the surface.
Dr. Foster and Smith sell a skimmer attachment that will fit onto the intake of your fluval canister filter, item CD-21212.
Alternatively, you can just a powerhead or two to vigrously agitate the surface of the tank and get those proteins stirred up so your filter and skimmer can remove them.