how the beep beep beep do i get rid of an oil slick at top of tank

[golf nut]

Originally Posted by kien

This is one of the reasons why my returns are at the opposite end of my peninsula. The return a the opposite end to the overflow helps to push the surface water towards the overflow.

This is 100% correct, I have seen way too many tanks with oils slick syndrome, always have the surface water move to the overflow box.

The foam in the overflow box doesn't sound quite right either, is it falling for quite a drop after going over the overflow? sounds like the overflow is acting as a skimmer.