I think everyone has been thinking of moray eels (Muraenidae), which are predatory. However, there are quite a few smaller eels out there in other families that would be safe with fish and larger inverts. I'm not sure if too many of them show up in the aquarium trade, but I know at least one group does - the garden eels. These are burrowing species that hang out in large groups on sandy bottom with their heads and part of their body poking out of the sand (thus looking like a garden of eels). They feed on plankton, which they snatch from the passing current. Many of the species are quite brightly coloured.
Tony