TANKS & FILTRATION

Sponge Filter

Air-driven foam block that is the safest biological filter for fry and shrimp.

Cycling stage Tanks

Unplug before servicingHands out of the water firstDrip loop on every cableTest before you dose30 ppm CO₂ ceilingCycle before stocking

Tanks

Specification
Drive	Air pump, 1–4 L/min
Foam pore size	20–40 ppi
Rated tank size	20–120 L per unit
Turnover	1–3× volume/hour
Power	Air pump, 2–5 W
Media cost	Negligible

Technical parameters
Shrimp tank	1 unit per 40 L
Breeding tank	1 unit per 30 L
Seed time to cycle	4–8 weeks
Cleaning interval	2–4 weeks
Air required per unit	~1 L/min
Lifespan of foam	2–5 years

A sponge filter is a block of open-cell foam with a lift tube through the middle. An air pump bubbles air up the tube, and the rising bubbles drag water through the foam. It is crude, it is nearly indestructible, and it is the filter of choice in breeding tanks and shrimp colonies.

The mechanism is entirely biological. The foam provides an enormous colonisable surface, and because the flow through it is slow, the bacteria that convert ammonia and nitrite are not washed off. It does no chemical filtration at all and only modest mechanical filtration — which is exactly why it is safe: there is no impeller to shred a fry or a shrimplet.

Its weakness is scale. Flow falls off as the foam clogs, so it needs squeezing out every two to four weeks, and a single sponge cannot handle a heavily stocked 120 cm tank. In larger tanks it is used as a secondary filter or as a seed filter to establish a colony quickly.

Safety and welfare

Fit a check valve on the air line between pump and tank. Without one, a power cut siphons tank water into the pump and onto the floor.
Position the pump above the waterline or use a non-return valve — air pumps are not waterproof.
Squeeze the foam in tank water only. Tap water chlorinates and kills the colony.

Using it well

Run a sponge filter alongside a new canister for six weeks when upgrading. It seeds the canister with bacteria instead of starting from zero.

Two sponges alternating: clean one, leave the other untouched. You never lose the whole colony in one go.

A shrimp colony will graze the foam surface constantly. That grazing is a good sign, not a problem.

Ideal for

Cherry shrimpFry and breeding tanksBetta tanksQuarantine tanksNano aquascapes

Note. Parameters are conventional ranges across the category, not the claim of any one manufacturer. Nothing on this page is for sale and no outcome for any plant or animal is guaranteed. Simulator output elsewhere on this site is a simplified model, not a measurement — confirm everything with a test kit and the equipment's own instructions.

Sponge Filter

Using it well

Ideal for

Works alongside