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You are here: myPetSmart.com > Pet Care Library > Articles > Facts About Under Gravel Filters And Gravel

Facts About Under-Gravel Filters and Gravel

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An undergravel filter is a basic method of performing biological filtration and some mechanical filtration.

Undergravel Filter
An undergravel filter is simply a slotted piece of plastic (undergravel plate) that sits on the bottom of the aquarium and lets water flow between the plate and the bottom of the tank. There is a tube that is attached to this bottom piece, known as the lift tube. Gravel is placed on top of the filter, and the filter, lift tube and gravel work together to provide filtration.

Water is pulled down to the bottom of the tank, through the gravel. And then the undergravel plate, then flows back up to the top through the lift tube. The water is again pulled down through the gravel. This allows beneficial bacteria to thrive on the oxygen laden water flowing through the gravel.

When using an undergravel filter you should purchase at least 1.5 pounds of gravel for each gallon your aquarium holds (a little less than that for tall hexagon-shaped aquariums). If you are using a power filter or some other type of filtration and no undergravel filter, you may purchase slightly less gravel. One pound of gravel per gallon in your aquarium should be enough.

Air Pump
Water is cycled through the aquarium with either a powerhead or an air pump. With an air pump, the air is pumped through plastic or silicon airline tubing to an air stone, which is suspended in the undergravel filter lift tube. The air forced through the air stone creates uniform bubbles in the tube, which changes the air pressure in the tube. This change in air pressure creates a vacuum that pulls water up and out of the tube, thus creating the suction underneath the gravel bed. It's kind of like drinking out of a glass with a straw.

Powerhead
Water can also be cycled using a powerhead instead of an air pump. In this case, a powerhead is placed on the top of the undergravel lift tube. It "powers" the undergravel filter by pulling the water down through the undergravel filter and up the lift tube where it is expelled onto the surface of the water. A powerhead can be used to increase the water circulation and improve the biological filtration of an underground filter. Gravel helps in filtration in several different ways.

  1. First, it collects loose debris as it's pulled down through the gravel.

     

  2. Second, it provides a breeding ground for beneficial bacteria in an aquarium since there's so much surface area for bacteria to grow on.

     

  3. Gravel also adds decoration to the aquarium.

Too much gravel will cause a problem with the water flow through the undergravel filter and can result in anaerobic bacteria, which is actually detrimental to the environment. An insufficient amount of gravel will not create a proper biological filtration bed. There needs to be enough gravel for the good bacteria to grow, and to make enough of a "filter" for waste to settle in.

Sand, Crushed Coral, Outdoor Gravel
Do not use sand, crushed coral, or regular outdoor gravel in your aquarium. Use only gravel specially sold for use in freshwater aquariums. Other types of gravel may release poisonous chemicals into the water. Sand and coral should be used only in marine aquariums (saltwater).

Lift Tube Algae Scrubber
To remove the algae from the undergravel lift tubes, a lift tube algae scrubber is shaped to work best. A standard bottle scrubber works also, but it must be used exclusively for the aquarium.

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