Advice? oil slick

Is it an actual overflow or syphon/transfer? A true overflow is as called. The water flows over the top of weir or pipe in the display tank. This allows that lighter compound that causes the refraction of light to circulate to the sump.

Protein skimmer should be removing the organics that cause the this unless your high in chemicals, then you need carbon.

 

I have this in a cube due to it not being a "surface" overflow. Its just got a grate section that goes under the water. I have a powerhead and the return pointed at the surface. I noticed this also in a qt tank when I was running one awhile back. I only had a sponge filter with a powerhead. I added a hob filter and it was gone in a day. As for the cube I have to figure out how to make it a real overflow.

 

More surface flow pointed toward your overflow should do the trick. I had same thing and added some surface agitation and got it go where the skimmer can get it.

 

I had this happen one time and I took a shop vac about an inch above the water and sucked it all up

 

I have seen this and as Jeremy said usually more surface agitation fixes the issue.