What Is the Proper Size for a Sand-Fluidized Bed?

Fluidized Bed Filter

• Fluidized bed filters draw water from the aquarium and pass it upward through sand in the filter. Because the water is under pressure, it lifts the sand from the bottom of the tube, keeping it suspended in the water during operation. When the aquarium water passes through the sand, the nitrifying bacteria in the water attach themselves to the sand. The cleaned water then proceeds upward through the filter and back into the aquarium. It is best to remember that these filters only reduce the total ammonia nitrogen (TAN) and do not filter out other components.

Total Ammonia Nitrogen

• TAN is important to the health of your fish. Both ammonia and nitrite are toxic to fish. Ammonia is a waste product from the fish and nitrifying bacteria convert it to nitrite. Nitrate, which is produced by good bacteria in the tank from the ammonia, is desired because it does not affect the health of the fish. Getting rid of the nitrifying bacteria will help to reduce the toxic nitrite levels in the water, leaving behind the good bacteria in the water to naturally process the ammonia waste from the fish. Sampling the water to determine the TAN is critical to choosing the amount of sand in the fluidized bed and the size of the filter itself.

Sand Amount

• The amount of sand in the filter is tied to the TAN level in the tank. According to "Marine Fish Culture" by John W. Tucker, TAN levels lower than 0.3 milligrams per liter (mg/L) require between 0.1 and 0.2 millimeters of sand in the filter bed. For TAN levels greater than 1 mg/L, use between 1 and 2 millimeters of sand in the filter. Check the filter at least twice a year and replace it so the amount of sand remains constant.

Filter Size

• The amount of sand in the fluidized bed filter needs to fill half the filter tube while the tube is under pressure. This ensures that there is enough room for the sand to remain in its suspended state in the water. Select a fluidized bed filter that will accommodate the needed amount of sand.