- 1). Measure the bottom of your tank with a tape measure. For an underground filter to work properly, it must cover the entire bottom surface of the tank. If you know how many gallons your tank holds, you can use that measurement to purchase a filter. Undergravel filters are sized according to the capacity of the tank, such as 20 or 29 gallons.
- 2). Place the uptake tubes in the holes at the back corners of the filter. These tubes provide the aeration the filter needs to function properly.
- 3). Set the underground filter in place on the bottom of the tank. Check to ensure that the filter extends to all four corners of the tank. Cover the filter with gravel to a depth of 2 to 3 inches.
- 4). Attach a piece of airline tubing to the connector on the uptake tube, and run the other end to your aquarium pump. You can also use a power head for the undergravel filter to provide even more powerful aeration. The power head sits inside the uptake tube, with the other end plugging into an electrical outlet. No matter which one you use, the concept remains the same: The power head or the air pump forces air through the filter and allows it to function properly.
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