The sponge is a mechanical filter and provides a bed for the bacteria to colonize. Biological filter size LARGE The sponge is a mechanical filter and the surface of all the bubbles or pockets in the sponge provide a bed for the bacteria of a biological filter. Large size XY380 Brand New How Does It Work? A sponge filter uses bubbles of air from an external air pump, or an integral water pump to draw water through the sponge. As water is drawn out through the lift tube or pump, it is replaced by water entering the sponge. The sponge is a mechanical filter and the surface of all the bubbles or pockets in the sponge provide a bed for the bacteria of a biological filter. The bacteria help to break down harmful waste (ammonia and nitrite). Sponge filters provide both mechanical and biological filtration. Most sponge filters are reasonably efficient filters for tanks. These filters are excellent for hatcheries, maternity tanks, shrimp tanks and fry tanks as the filter produces minimal current and has almost no way to suck the fish through the filter. Also, the air infusion chamber produces minute bubbles which increase the oxygen solubility. What Maintenance Does It Require? The sponge will need to be rinsed every time it becomes dirty, about once every 2 to 4 weeks. It’s helpful to reserve the beneficial bacteria by washing the sponge gently with water from the fish tank instead of tap water. After 3 to 6 months’ usage, the micro holes of sponge is gradually clogged by the tiny debris, the beneficial bacteria won’t have as much living space as before. That’s when the sponge filter starts to lose its efficiency. So it is also recommended to replace the sponge once every three to six month or when the sponge gets worn out.