You would be best using Live Rock as said, IMO but if you're bothered about hitchhikers, get reef bones (dead live rock that's been dried out). It's cheaper, and will work just as well for filtration, minus the critters, good or bad.
As corals don't produce much waste in the form of ammonia, etc, you won't need much either. A few KGs of smallish nuggets will form ample biological filtration for a frag tank.
You could use an internal filter, but it'd have to have ceramic rings in anyway for biological filtration, and so what's the point in an extra electronic device when you can use the flow in the DT around a few KGs of LR? Also, there's the danger of the "ticking time-bomb" with enclosed filters like internal and external filters, where if the power goes, the aerobic bacteria quickly consume all available oxygen in that small body of water then die. When power resumes you've lost your bio filter, or a good chunk of it.