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I wanted to cover this topic for years, but have not had the opportunity until now. I think there is no other filter system that is more misunderstood then the undergravel filter. The undergravel filter is one of the older types of filters still in use. An undergravel filter is still an excellent biological filter system. I stress biological because the undergravel filter system is only biological, not mechanical or chemical. All three types of filtration are needed to have a complete filtration system. The undergravel filter uses the gravel as a biological filter bed. Lets go over the mechanics of how it works. The undergravel filter is a perforated plate(s) that goes on the bottom of the tank before the gravel . Attached to the plate(s) is a lift tube usually 1ä-1-1/8ä diameter of rigid tubing In the tube at the bottom (about 1ä from the plate(s)) is an air diffuser. This creates smaller air bubbles that rise to the top of the air lift tube pulling water from underneath the undergravel plate(s).

On top of the undergravel filter plate(s) is your gravel which should be at least 1-1/2ä to 2ä in height (or 1-1/2 or to 2 pounds per gallon of gravel). The air passes up the rigid tube which creates a suction that causes water to be drawn through the gravel. This water movement creates oxygen which feeds the bacteria that has imbedded (latched-on) to the gravel which in-turn converts harmful ammonia and nitrites to less toxic nitrates. The biggest mistake comes from people misunderstanding how or what the filter does. I think the biggest misunderstanding is that if you have an undergravel filter, you do not have to do gravel cleanings. WRONG! Please remember that you want water to pass through the gravel which in-turn feeds the bacteria with oxygen. If the gravel becomes clogged and water cannot pass through it freely, the bacteria will start to die off. This may cause the bacteria to become anaerobic instead of aerobic bacteria. This function probably will create massive crashs in your aquarium. Doing regular gravel cleanings will prevent this from ever happening. A good rule of thumb is to clean about 70% of your gravel every 4 to 6 weeks when you are doing your regular water change. Another mistake is when people strip down their aquarium to clean under the undergravel filter. This will kill the bacteria and renders the filter useless. Remember it takes 4 to 6 weeks to culture full bacteria counts in an aquarium. I hope this might clear up how an undergravel filter really works, and what part in the success of a healthy aquarium it plays. To have a complete filtration system an undergravel filter is an excellent choice, but not by itself. For a complete filtration system you should have a mechanical and chemical filter (e.g. hang-on or canister type of filter) as well as a biological filter.

Till next time remember there is no quick fix or magic pill ,do regular maintenance and enjoy your tank.

Marine Manager

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