How They Work
Water flows over open bio media (bio balls, ceramic rings) exposed to air. Bacteria get unlimited oxygen — biological capacity is 2-3x submerged filters.
Water then drips into a sump or returns directly to tank.
Best Use Cases
Discus tanks — high stocking, sensitive water quality.
Predator setups (oscars, arowanas, large catfish).
Reef tanks (built into sumps).
Setup
Built into a sump or as an external box behind the tank.
Returns water via gravity flow + booster pump or full sump pump.
Total system cost: $200-1000.
Trade-offs
CO2 loss is significant — bad for high-tech planted tanks.
Noise from water trickling over bio balls.
Footprint: requires sump space below tank or external chamber.
- ✦Keep bio media moist — never let dry out
- ✦Reduce flow for CO2 retention if planning live plants
- ✦Pair with mechanical pre-filter to keep bio media clean