Sillyfishies
Aquarium Advice Addict
Powerheads aren't a filter...?
Your rock is your main source of biological filtration.
A sump, will normally be mechanical, unless it's a nano or you are unable to a sump. In which case it would be a HOB or Canister. Hob being more common.
Without water flow the rock wouldn't be filtering much. You can have a very successful tank with no real mechanical filtration. I would just use rock, powerheads, and a skimmer.