In the hobby we make the distinction (semantically) between "sponge filter" and "internal filter" as follows (I believe):
Sponge filter: Any of various filtration devices where the sponge is directly exposed to the tank and inhabitants with the water moving device imbedded internal to the sponge, thus pulling water through the sponge and out, via the internal water moving device. The classic example of a sponge filter is a simple clear plastic stiff tube with holes in it that has been glued parallel to an airline and is sealed at the bottom just below where the airline connects in. The air pump dumps air into the bottom of this tube and as the air bubbles rise, they fit tightly against the inside surface of the tube and push water upward and out the top of the tube. This creates a vacuum which pulls more water in via the holes drilled in the sides of the tube. The entire tube and airline are enclosed in an open-cell sponge through which a hole or slit has been made to facilitate sticking the tube down through it. The sponge helps to further ensure that the air bubbles go straight up the tube and not out any side holes and of course as water pulls through the sponge and into the airlift holse, the sponge filters the water in both a mechanical (debris-catching) manner and eventually (if there is an ammonia source) in a biological manner.
Internal filter: Although this term could be wide enough to include the sponge filter described above, in practice we would be more likely to be referring to a filter of the following type: Any of many styles of boxes that hold any type of filter media and have any of several methods for moving tank water throught that media and back out into the tank. The box is usually fully submerged to be called an internal filter. For many many decades in the hobby the method of moving water through an internal filter was via an air pump and bubbles or airstones, which move water in a similar way to that described for the sponge filter above. In just the recent years, the water moving method has moved in majority to actual small water pumps or "pumpheads." Internal filters are typically more complex than a sponge filter in that they often have media trays (or holders of some sort) for more than one type of media. In fact usually all three typical media types are represented: biomedia, chemical media and fine mechanical media. The flow from an internal filter can also serve as the main circulation device for the tank and in fact in most small tanks its the case that this is the only circulation device and there are no extra powerheads moving water around. The internal filter outlet is also often used as the surface disturbance device for promoting good gas exchange with the air. The media volume of internal filters typically overlaps with hang-on-back filters but is typically less than the largest of these of of almost any of the external cannister type of filters.
(I should be trying to write for a tropical fish wiki project, shouldn't I?
) (Don't worry, you'll be rid of me soon, my right wrist will have to be put in a cast
)
~~waterdrop~~
