How to Remove Nitrogen From Water for Fish Tanks

All freshwater and marine aquaria experience the nitrogen cycle. In this process, aquarium hobbyists add nitrogen to the aquarium in the form of fish food, which fish consume and excrete as waste. Filtration removes the resulting waste containing ammonia and nitrates from the water and helps maintain a balanced environment for the fish and plants. Mechanical and chemical filtration, often working together in the same filter, removes nitrogen by flowing dirty water through charcoal and nylon floss to absorb chemicals and trap particles. Biological filtration uses nitrogen-eating bacteria, cultivated in an undergravel filter system, to remove harmful ammonia and nitrates from the water.

Things You'll Need

  • Aquarium
  • Internal box filter
  • Activated charcoal
  • Nylon floss
  • Colander
  • Undergravel filter kit, including airlifts, caps and air tubes
  • 2 airstones
  • Plastic tubing
  • Air pump
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Instructions

  1. Mechanical/Chemical Filtration

    • 1

      Measure out enough activated charcoal to fill the box filter about halfway. Pour into the colander and hold under a stream of cold water to eliminate as much dust as possible.

    • 2

      Remove the lid from the box filter and add the charcoal, half-filling the box. Smooth into an even layer.

    • 3

      Fill the rest of the box with nylon floss. Put the top of the box filter back on securely.

    • 4

      Attach a length of plastic tubing, long enough to reach from the filter when inside the aquarium to the external air pump, to the top spout of the box filter.

    • 5

      Place the box filter inside the aquarium. Position toward the back of the tank or behind a clump of plants to hide it from view. Hold the filter down until all air bubbles have escaped and the filter rests on the bottom of the tank.

    • 6

      Attach the other end of the tubing to the air pump. Plug the air pump into an electrical outlet and turn the pump on. Air will push water through the charcoal and nylon floss, trapping both particles and dissolved nitrogen.

    • 7

      Replace the charcoal and floss with new material when the floss appears dirty or the aquarium water seems cloudy.

    Biological Filtration

    • 8

      Lay the plastic filter plate in the bottom of an empty aquarium. The perforated plate should cover as much of the floor as possible while remaining flat. The one or two holes for the airlifts should be at the back of the aquarium.

    • 9

      Insert the airlift tubes into the holes in the plastic filter. Attach the two narrow air tubes to the inside spouts of the airlift caps, then attach each airstone to the end of the narrow tube with a short length of flexible plastic tubing.

    • 10

      Insert the airstones into the large airlifts and attach the caps firmly.

    • 11

      Attach a suitable length of flexible plastic tubing to the top of each cap. Connect the two ends with a T-connector. Attach the T-connector to the air pump with another piece of plastic tubing. (If there is only one airlift, omit the T-connector step.)

    • 12

      Set up the aquarium with the proper substrate, plants and water. Make sure that the filter is covered with at least 2 inches of gravel, and the gravel particles are at least 1/8 inch in diameter to ensure proper water flow through the filter.

    • 13

      Turn on the air pump to begin air flow through the filter. Allow the bacteria colonies to develop for a few weeks before introducing fish, or "seed" the colony with gravel from an established aquarium.