Both filters move water through media and grow bacteria. The differences that matter are intake safety, flow rate, and what happens during a power outage.
Sponge filters
An air pump pushes bubbles up a lift tube; the rising bubbles pull water through a sponge. Cheap, nearly silent, and the sponge is simultaneously your mechanical and biological media.
Strengths:
- Nothing gets sucked in. The intake is the sponge. Shrimplets, fry, and betta fins are all safe.
- Gentle flow. Bettas can't fight a current and shouldn't have to.
- Huge bacterial surface relative to cost.
- Great in a quarantine or breeding tank — you can keep a spare sponge running in your main tank and move it over instantly cycled.
Limits: no chemical filtration stage, doesn't polish water to glass clarity, and you've got an air pump on the shelf making noise.
Hang-on-back filters
A powerhead pulls water up an intake tube, through cartridges, and back over a waterfall lip.
Strengths: much higher turnover, real multi-stage filtration including carbon or resin, better surface agitation, and clearer water in a heavily-stocked community tank.
Limits: the intake is a hazard. Shrimplets and fry get pulled in, and long-finned bettas can get pinned against the strainer. The fix is a sponge pre-filter slipped over the intake — which, note, turns part of your HOB into a sponge filter anyway.
Pick by what you're keeping
- Shrimp tank — sponge filter. Not a preference; shrimplets are small enough to go through almost any strainer.
- Betta — sponge filter, or a HOB turned well down with the outflow baffled.
- Fry / breeding — sponge filter, every time.
- Community tank, messy fish — HOB, with a sponge pre-filter on the intake.
- Quarantine — sponge filter, so you can seed it from an established tank.
The power outage argument
A sponge filter runs on an air pump. Put that pump on a small battery backup and your biological filter keeps running through an outage for pennies. A HOB stops dead, and if it's off long enough the bacteria in it start dying — then it dumps that decay into your tank when power returns.
Running both is legitimate
Plenty of well-run tanks have a HOB for turnover and polish plus a sponge filter for redundancy and safety. They're cheap, they're quiet, and the sponge becomes your instant-cycle spare for the next tank. Nobody has ever regretted the extra sponge.