Since the frame is rusting, it must be made of steel. I don't think it is possible to paint steel well enough to withstand constantly being wet. If you can keep the rim dry, like using it without a top on the tank, just carefully cleaning off the rust, priming and coating with rust resistant paint should work. Look for "naval jelly", I think it is called, to help with removing the rust. I would also want to use spray can rustoleum paint too, but that would require a lot of masking and draping paper around the area to keep the paint where you want it.