Oily sheen from filter pump?

[Shadey]

I set up a new 2.5G fluval spec about a month ago. After it had been running for just a little while, it developed this rainbowy oil sheen in the compartment where the pump lives. I took the pump back to the store I got it from, and they replaced it. I cleaned up the existing oil, but it came back. There is a colorless kind of oily sheen in the main tank part as well.

Suggestions?

Thanks!