An undergravel filter is really just a plastic plate that covers the bottom of the aquarium. The plate has many small holes or slots in it. The exact physical appearance varies from one brand to another, but all work on exactly the same principle.
The undergravel filter plate is covered with aquarium gravel. Located at each back corner of the plate is a lift tube that extends from the plate to the top of the tank.
The aquarium water is drawn up the lift tube along with bubbles from an air stone at the bottom or by a small motor, called a powerhead, at the top. This pulls water from under the plate, which in turn draws water down through the gravel.
As the water passes across the grains of gravel, the nitrifying bacteria living there use the dissolved oxygen in the water to metabolize the ammonia and nitrite. The grains of gravel offer a huge total surface area for the bacteria to colonize, and the steady water movement ensures that enough oxygen will be available to support the large colonies.
As a result, the aquarium will house more than enough nitrifying bacteria to complete the nitrogen cycle, and the fish will have a healthy environment.
In addition to being a biological filter, the undergravel filter acts as a mechanical filter because it catches solids as they pass into the gravel bed.
Unfortunately, this actually works against the biological filtration process because as the spaces between the pieces of gravel fill with particles, the flow of oxygenated water past the bacteria is reduced. Over time, the effectiveness of the biological filtration is significantly impaired.
I don't filter anything under 10 gallons but if you want to have a filter, I would look into something other then an undergravel filter. I have never used any of these myself but maybe look into either a sponge filter or Penn Plax's small world filter. Both require the use of an air pump. Teta also has one called a tetra 3i that is for tanks up to 3 gallons and that one also uses a air pump.
This is the small world filter:
Note: Pic from amazon so those goldies are NOT mine.
- Helps keep your small aquarium healthy and looking beautiful.
- Specifically designed for small aquariums up to 5 gallons.
- Fits almost any small aquarium shape or size, and is compatible with any air pump.
- Clears discoloration and removes harmful contaminants & odors.
- High quality carbon/zeolite cartridge for chemical filtration and a foam block filter for mechanical & biological filtration
Sponge filter for up to 5 gallons - sponge filters are supposed to be one of the best for biological filtration. Plus they are gentle enough to use with bettas and fry