The Aquarium and Pond Active Online Publication

The Denitrator

What is a Denitrator?
Why Do I Need One?
How Does it Work?

Denitrator Figure

By Tony Griffitts

A denitrator is a biological filter that removes nitrate (NO3) from the aquarium. A denitrator filter uses anaerobic bacteria to brake down nitrate into nitrogen gas (N2), which escapes into the atmosphere, the result is nitrate free effluent.

Nitrate is toxic to many marine invertebrates even at low levels. The deaths of some of the more delicate marine fishes have also been linked to high nitrate levels in the aquarium.

A well designed denitrator can maintain nitrate levels near 0ppm. With low nitrate levels you reduce the frequency and the volume of water you need to change annually. A denitrator drastically reduces the time and money needed to maintain a reef tank. Also, you can stock the aquarium with a lot more fish and invertebrates than you can without a denitrator.

The denitrator works by sending water through the filter at a slow flow rate. The bacteria at the first part of the filter are aerobic (use oxygen). The aerobic bacteria use up all the free oxygen in the first stage of the filter. In the second part of the filter there are anaerobic bacteria, which exist in the oxygen depleted environment by breaking the oxygen atoms of the nitrate in order to respire. At the end of the process all the oxygen has been remove from the nitrate molecule and the end byproduct is nitrogen gas.

The denitrator takes about 4 to 6 weeks for the bacteria to colonize the filter in sufficient quantities for the filter to start reducing the nitrate in the aquarium. The filter will not be fully mature until it is 3 to 6 months old. When you first set up the filter run the aquarium water through it at a swift rate for about 2 week. This will give bacteria time to colonize the filter. After 2 weeks cut down the filter flow rate to about a drop or two a second.  At this time, start to add about 5ml of bacteria food a day to the filter. Using a dosing pump plugged into a timer that comes on 4 times a day is highly recommend for adding the food on a regular bases.


Once each day check the effluent water coming out of the denitrator for nitrate and nitrite (NO2). If your tests indicate any traceable amounts of nitrate and a noticeable amount of nitrite you will need to slow the flow rate down. What is happening here, is the anaerobic bacteria is not receiving enough exposure to the nitrate to brake off all the oxygen atoms from the nitrate. Thus with only one oxygen atom broken off the nitrate you now have nitrite.

If reducing the flow rate does not seem to help, increase the amount of food added to the filter. The increase in food may increase the bacterial activity and nitrate reduction. If your flow rate is to slow and/or you are feeding the bacteria too much, your filter will start producing hydrogen sulfide. You can easily detect hydrogen sulfide by the rotten egg smell. Increase your flow rate and test the effluent the next day for nitrate and nitrite. Over time you will have to increase the flow rate as the filter matures and becomes more efficient. When your filter is fully mature your flow rate should be a steady stream.

With experience you will be able to dial in the filter and know exactly how much food to feed, and how fast the water should flow through the denitrator. A properly sized denitrator will be able to handle all your nitrate problems, even in the most heavily stock reef tanks.

First published in 1991, Updated Re-Published - 2000

Aquaworld Sponsor