How Do Under Gravel Filters Work?

[mike_nofx]

I have an under gravel filter in my tank right now with a power head in the corner pumping the water from under the gravel to the top of the tank.
What actually filters the water? is it just the piece of plastic with the holes in it? i set the tank up a while ago when i didnt know much/anything (still dont!) about aquarium filters.
This doesnt seem adaquet filtration if it is only the plastic with holes in it...
I didnt notice any wool or sponge or anything..

The tank is very small - 28L so maybe its ok for this size?

Thanks

 

[FBF]

y'know i didnt think people still bought those. very old tech. even when i started out fishkeeping no one had those

 

[mike_nofx]

Hehe yeah i know now.... But i did buy it brand new only like 4 months ago... AquaOne still sell them.
My Tank
Being a total noob when i bought it, i didnt know it was an old style filter. Im buying a new tank soon, and this will be a quaranteen/hospital tank.
Tank i want - 120

 

[srolfe]

The tank water is drawn thru the gravel by the powerhead. The gravel becomes the filter medium.

 

[riverman444]

The water is pulled through the gravel and under the filter grid. Most external filters have bacteria in the filter itself that breaks down chemicals in the water. Under your filter grid there are a bunch of bacteria and microorganisms that do the same. Not ideal for planted tanks, however, but a decent filter nonetheless.

If you want to understand the mechanics of a undergravel filter.....try this.

Fill a glass with water and get a straw. Put your finger over the end of the straw and stick it in the water. No water should come in to the straw. Now, let your finger come off of the end of the straw. See how the water shoots up? That's how your filter works.........air lifting in the tubes pulls water down.

 

[Neal]

My undergravel filter actually has a filter mat in it, so it goes through the gravel and the mat. I'm sure it works at when I clean the tank out, the mat is extremely filthy...