What is an under gravel filter?

Hello! an under gravel filter came with the aquarium I have and I am wondering exactly how they work and if I have to do anything special in my tank for it? any info on this would be great!

This is totally one for John to answer. He loves those things.

All twentysomeodd of my aquariums have under gravel filters. They're cheap and they work great if you don't overload the aquarium and if you do regular partial water changes. Under gravel filters work by drawing all of the aquarium water through the gravel substrate, which should be at least two inches deep. Eventually, all the bacteria and inert particles are concentrated in the gravel. This nurtures the bacteria and slightly lowers the pH of the aquarium water. If you use a syphon with a large intake tube, you can clean about half of the gravel with every partial water change.

Don't use under gravel filtering if your aquarium has a very large biological load like the tanks in a fish store do.

okay! I only have a 5 gallon and I usually only have 2 fish. I definitely do not have 2 inches of gravel so I will have to make a trip to the pet store. My old filter was way different then this one so I had absolutely no idea how it worked. Thanks again. Does this type of filter also bring oxygen into the water?

Yes. The air bubbles bring in oxygen and keep the water agitated so it can absorb more oxygen from the air. A few years after I started aquarium keeping, under gravel filters were introduced, and they were the bees' knees.

Since you have a small aquarium, you may want to supplement the gravel substrate with something like Flourite. I never get the spelling right, but it's a clay based substrate that really helps your plants grow. The stuff is also full of dust, and it may take your aquarium a week or two to clear. Instead, you can mix in a smaller amount of pure laterite, and the tank may clear sooner. After rinsing clay substrates in a bucket and losing about half the material, I just started dumping it in unrinsed. The aquarium clears about as fast, you don't waste any expensive clay stuff, and it's no big deal if the tank doesn't yet have fish. In fact, I've used it in established tanks with no ill effects.