Much is written about nitrification – the conversion of ammonia to nitrite, and then to nitrate – by autotrophic nitrifying bacteria. Nitrification is a vital process in the aquarium environment. Without nitrification ammonia would build-up to toxic concentrations and for the fish to survive aquarists would have to perform near continuous water changes.
Nitrification is also a part of the natural nitrogen cycle. In fact, “cycling” an aquarium is a term in the hobby’s jargon which means to establish the process of nitrification. However, nitrification is not a cyclic process because the end product of nitrification is nitrate. If nitrification were cyclic the end product would be ammonia (the starting product) as a cycle means to come back around to the starting point. Instead, nitrification is a one-way process producing nitrate.
Most hobbyists are content with efficient nitrification and counteract the eventual build-up of nitrate by simple water changes. However, nitrate can be removed by biological processes. This post will explain the denitrification process along with the caveats associated with trying it in your aquarium. Interest in denitrification in the aquarium environment is on the increase because it can be used to keep nitrate at a low concentration without water changes and, in some case, to moderate the drop in pH that accompanies nitrification.
First, some terms and processes need to be understood. Nitrification is an obligate aerobic, oxidizing process. This means that it can only occur in an environment which contains oxygen and the process produces electrons, respectively. In general, denitrification is the process of bacteria converting nitrate to other substances. It is an anaerobic, reducing process. This means that it occurs in environments without oxygen, and the process accepts electrons, respectively.
In actuality there are several processes that can occur in an aquarium which use nitrate. In order to understand exactly what is happening in your aquarium it is important to understand the different types of nitrate conversions possible as some can produce toxic products. Denitrification is defined as the transformation of nitrate to dinitrogen (N2). Dinitrogen is a gas that is harmless and will bubble out of the system. Between the starting product (nitrate) and the dinitrogen end product there are three intermediate products. These are, in the order is which they are produced, nitrite (NO2-), nitric oxide (NO), and nitrous oxide (N2O).
There two other processes that can occur in an aquarium to get rid of nitrate which are 1) dissimilatory nitrate reduction and 2) assimilatory nitrate reduction. They are not desirable because the end product of both these processes is ammonia. Their pathway is the exact opposite of nitrification. Nitrate is reduced to nitrite, which is reduced to hydroxylamine, then to ammonia. Obviously, re-generating ammonia from nitrate is not what one wants to do in their aquarium.
Bacteria are responsible for all these processes. Furthermore, one thing that all the above processes have in common is that an intermediate product generated in the reactions is nitrite. This is another substance that is not desirable to have in the aquarium. You do not want nitrate going back and staying as nitrite.
As stated before denitrification, like nitrification, is a multi-step process with many intermediate compounds produced before the final product is generated. In most cases these intermediate products are toxic. Thus if denitrification does not proceed to completion, the fish will be in a much worse environment (in terms of water quality) than before.
Another key difference difference between nitrification and denitrification is the type of bacteria that perform the processes. Nitrification is done by what are called autotrophic bacteria. This term means the bacteria get the carbon they need for cell growth from carbon dioxide. Denitrifying bacteria are heterotrophic bacteria. This means they get their carbon from organic carbon sources such as methane, sucrose or glucose (to name just a few).
So what does all this mean to the hobbyist? It means that in order to have successful denitrification in the aquarium a few things have to be provided by the hobbyist. This is much different than what is needed for nitrification. Now, you might say “how can this be different when we have to provide a biological filter for nitrification? The response is that, nitrification will proceed in an aquarium with fish in one fashion or another whether the hobbyist does anything or not. It might not be real efficient but it will proceed. Further, the general conditions preferred by the nitrifying bacteria are pretty similar to those preferred by most fish. Both like warm water, pH in the 6 to 8.5 range and, most importantly, clean water (meaning little turbidity) with lots of oxygen. This is also what can be termed a high redox potential environment.
Denitrification, on the other hand, will not proceed unless the aquarist actively provides, and continues to promote, a special environment that has a low redox potential. First, an anaerobic area must be provided. Oxygen will kill the denitrifying bacteria, halting the process and necessitating the restarting of the filter (meaning the bacteria have to become re-established). This is not an easy thing to do. How does one create such an environment? Where should you put it in the tank? How do you assure that it remains anaerobic? Unfortunately, I have no answers readily available. One option is to chemically strip the oxygen from the water but this is expensive and involves chemicals that might not be compatible with the rest of the system.
Another problem is that you have to provide a constant source of the organic carbon required by the bacteria. This means mixing it up and delivering it at the correct infusion rate that will change over time. There is not enough naturally occurring organic material in an aquarium to keep the process going. Exactly how to do this for a home aquarium is not well documented.
Lastly, there is the problem of balancing the system so the intermediate products do not accumulate. The first product produced when nitrate is reduced is nitrite. As mention before, this is a toxic compound and it should not build-up in an aquarium. In many denitrifying systems, nitrite is a constant problem. The reason is usually an insufficient (incorrect) assemblage of microorganisms. The problem is providing and maintaining the correct assemblage of microorganisms. How to do this is again not well known. What is known is that redox potential can be an important indicator. Denitrification proceeds well within a range of redox values but if the redox gets too low than sulfate will be reduced to hydrogen sulfide that is a poison. Usually the cause of a redox value being too low is a lack of substrate for the bacteria to utilize so the bacteria switch to another substrate. Thus, one has to carefully control the mix of nitrate and organic carbon into the denitrifying filter. Too much water flow and oxygen may enter the filter, too little water flow and there won’t be enough and sulfate reduction can occur.
Does all this mean that denitrification should not or cannot be done by the hobbyist? The answer is no. But the hobbyist should know beforehand about the myriad of potential problems associated with denitrification. Constant monitoring of the water quality is needed so the process can be tracked and if needed, corrective action taken. Detailed notes should be taken so that reasons for successes and failures are known. In this way you can repeat the positive but not the negative.
Finally, there are a few commercially available systems for denitrification. They range from simple to complex. Some ‘systems’ are basically nothing more than a pile of granular media that is allowed to sit in the sump of a tank without being disturbed. The manufacturers of this say that the inside area of the pile will go anaerobic and denitrification will commence. My experience with these systems is that they do nothing towards lowering the nitrate levels in the aquarium.
Other systems for denitrification involve a sealed chamber through which a portion of filter water is directed at a very slow flow rate. There is a separate tube in which a pellet or liquid organic carbon is added periodically. These systems must run for a long time before the water exiting the system slow any signs of a lower nitrate concentration. Unfortunately, these systems are quite finicky and never produced much nitrate-free water and tend to clog.
Denitrification in freshwater, saltwater or reef aquaria can work but denitrifying filters are not worry and maintenance-free. In another post, I will discuss less troublesome alternatives to denitrification that can remove nitrate from aquarium water.