Filtration
Sponge filter
The default choice for breeding tanks, shrimp tanks, quarantine tanks and anything needing very gentle flow. It moves comparatively little water and does nothing for surface skimming, so it is a poor fit for heavily stocked or large tanks on its own.
Short answer
An air pump draws water through a foam block, which provides both mechanical trapping and a large biological surface. Cheap, gentle, and impossible for fry or shrimp to be drawn into.
What it does
- Houses nitrifying bacteria on a large foam surface
- Traps suspended particles
- Agitates the surface via rising bubbles, aiding gas exchange
Strengths and where it fits
Strengths
- No intake hazard — safe for fry, shrimp and long-finned fish
- Very gentle flow suited to bettas and blackwater species
- Inexpensive, and easy to keep a spare running in an established tank as a cycled backup
- Nothing to break except the air pump
Good fit for
- Breeding and fry tanks
- Shrimp tanks
- Quarantine and hospital tanks
- Betta tanks
- Small nano tanks
Limitations and where it is the wrong choice
Limitations
- Low flow rate; inadequate as the sole filter for a heavily stocked tank
- No chemical filtration stage without modification
- Air pump noise, and bubbles that spray salt creep and condensation
- Visible in the tank, which some keepers dislike
Poor fit for
- Heavily stocked community tanks as the only filter
- Large tanks
- Tanks needing strong current
Sizing
- Match the sponge to the tank volume as the manufacturer indicates, then err one size larger — a bigger sponge holds more bacteria and clogs more slowly.
- Two smaller sponges in opposite corners give better circulation than one large one in a corner.