There is an interesting wet/dry filter by Resun that works on top of tanks. Maybe using their idea as a lead could help you.
These units move the water from the tank through a standard
powerhead (I would use a pre-filter there), pump the water through attached hose to a horizontally placed, bottom only perforated acrylic rigid tube (
PVC would work as well) that feeds a tray with ceramic rings and is headed back into the tank by gravity.
You would need to figure out
GPH requirements to determine adequate tubing diameter and pump capacity.
Hope this helps
Pepe
Santo Domingo