Originally Posted by Otterfun
How is a sponge filter different from an airstone? do they have the same function as in getting oxygen via the air pump only one with and one without sponge? I am confused.
The sponge filter is designed so that as air runs through it and toward the surface, water is drawn through the sponge. The porous nature of the sponge allows for bacteria growth - the water is essentially dragged through a serving line of bacteria, who feast on all of the nastiness in the water. This is biological filtration.
It also adds a bit of mechanical filtration (i.e. poop catching) as the water is drawn to and through the sponge, because moving water obviously will drag solid waste with it to some extent. This will get trapped on the sponge.
Cleaning a sponge filter is easy - just dip it into a bowl of tank water (like when you perform a water change) and give it a few squeezes.