Oily Film

[untamed]

Yes...I had this same problem on my tank. Basically, the tank itself has become a giant protein skimmer with no collection cup!

For fun, you can stick your arm straight into the middle...go past your elbow...pull your arm straight up and the gunk will coat your arm as you pull it out! Not a solution, but good to gross our your significant other!

You need to draw the water from the surface into a skimmer... or break up the surface with a PH so that a skimmer can get it later. IME, I found it nearly impossible to keep a surface feed working with my HOB skimmer...unless you have perfect evaporation control, the surface will change height and you have a lot of problems.

All that until I got a sump. That solved this problem completely as the overflow into the sump comes off the surface of the tank.