hmmm, so we're looking for predators you can house in a 30g tank.....
I'll keep thinking, but all that really comes to mind is the golden moray eel. There is a post on here somewhere about it, complete with pics. I'll try to dig it up and post a link. a dwarf lion might work, some (even though dwarfs) still get very large some upto 13". you could do a hawkfish (smaller variety eg; longnose, flame or falco), there are the small puffers, and probably a few others, I'll keep thinking on it.
what exactly is needed for "predator filtration"? i've got a nice canister filter and protein skimmer. anything else?
depending o the canister (I've never been crazy about them so haven't kept up with what does what), that is probably fine, you just have to keep up with the mess of predators.