Understanding Nitrate

Nitrate in the aquarium is one of those aspects of reefkeeping that seems to cause many people problems and yet it is something that is relatively easy to deal with. In this article I will cover some of the aspects of nitrate in the aquarium, what it is, how to avoid it and what can be done if you are having nitrate problems.

Nitrate and the Nitogen Cycle
Nitrate is a molecule that contains one nitrogen atom and three oxygen atoms attached, this is written as NO3- and is the end product of the nitrification process that occurs in the aquarium. Nitrification is the process where decomposing material and waste products from fish and other organisms are broken down by bacteria

After the ammonia is broken down to nitrite, further bacteria break it down into nitrate:

The bacteria that convert ammonia to nitrite and nitrite to nitrate are known as aerobic bacteria and live in oxygen rich environments in the aquarium and on filter media. The nitrate then needs to be further broken down, in the aquarium we want the nitrate to be converted to nitrogen gas and this process is known as denitrification. The bacteria that convert nitrate into free nitrogen gas are known as anaerobic, they do not gain oxygen in the same way that aerobic bacteria do. Anaerobic bacteria gain their oxygen by reducing nitrate molecules and taking the oxygen atom from the nitrate molecule for respiration. Anaerobic bacteria will only live in anoxic areas of the aquarium, that is areas that only have low concentrations of oxygen. So if we want to maintain plenty of anaerobic bacteria in our aquariums to convert all of the nitrate into free nitrogen gas then we need to provide plenty of areas that have low concentrations of oxygen.

Sources of Nitrate
Nitrate comes from the metabolic waste products of organisms and decomposing organic matter, ultimately the main source of nitrate in the aquarium is the food that you put in, the more food that you put in your aquarium the more nitrate is going to be produced. The key to having a system with low nitrates is ensuring that you are not adding more nitrogenous compounds than are being removed. In the sea nitrate concentrations vary with depth and habitat but are generally found to be less than 0.1ppm on coral reefs. Within the reef aquarium we should always strive to maintain concentrations below 1ppm, with the test kits that are available to hobbyists you can measure nitrate down to 0.2ppm (Salifert) and provided that your system is set up well then it is not difficult to maintain nitrate levels below detectable limits on these test kits.

Reducing Nitrate with Bacteria
As mentioned previously the bacteria that break down nitrate live in anoxic (low oxygen) areas of the aquarium, primarily these areas consist of the lower areas of sand beds and the inner areas of live rock. Live rock consists of ancient dead coral skeleton and as such it is porous, on the surface of the rock a wealth of flora and fauna live. As water passes over the live rock some water seeps through its porous structure, as the water enters the rock the tiny organisms and bacteria living on the surface and the outer layers use up the oxygen in the water, hence the water that penetrates deeper into the rock becomes depleted of oxygen and anoxic conditions arise. These low oxygen environments that are created within the live rock become the ideal habitat for anaerobic bacteria to live.
Sand beds work in the same way, the upper layers of the sand bed have a multitude of small organisms and aerobic bacteria colonising them, as the water seeps down into the sand bed oxygen becomes depleted and anoxic conditions develop. Usually deep sand beds are used for nitrate reduction but these anoxic areas can also occur in shallow sand beds. Provided that fine grain substrates are used anoxic areas can develop within 2-3 cm of the surface of the substrate depending on grain size, water flow and the abundance of organisms and organic material present.

It is necessary to maintain a good level of water movement around living rock and sand beds, without good water movement you are restricting the areas that both aerobic and anaerobic organisms can live. Very often aquariums that are suffering from nitrate problems can be improved by increasing the water movement in the system if the previous levels of water flow have been too low.

It is essential to ensure that there are enough areas in which anoxic conditions can arise, deep sand beds are a cheap way of ensuring this but many systems these days incorporate the Berlin style of filtration which incorporates live rock and a shallow sand bed.

The Berlin system is my preferred filtration method but if you are running this type of system then you must ensure that you have ample live rock present. There are a range of recommendations for the amount of live rock that you should use in a reef system, lower estimates are at 0.7kg of live rock per gallon of aquarium water, upper estimates are around double that at 1.4kg of live rock per gallon of aquarium water. Personally I run my systems on at least 1kg of live rock per gallon of aquarium water and this seems to work fine and maintain nitrates at undetectable levels. Unfortunately far too many people seem to scrimp on the live rock when they are setting up systems, this is usually due to the high cost of the rock, however if you are setting up a Berlin style system then it is essential to ensure that you put in enough live rock or you are almost guaranteed to have problems with nitrate. You can reduce the costs of the initial set-up by using reef bones for up to half of the rock structure in your aquarium and live rock for the other half. Reef bones has the same porous structure as live rock and it will be colonised by bacteria and other organisms from the live rock. Again it is essential for there to be good water flow around your live rock to enable it to function properly and to prevent dead areas from occurring, creating an open rock structure with high water flow will enhance your systems ability to deal with nitrogenous waste.

Protein Skimming
Protein skimming is an excellent way of removing organic matter from the water and helping to lower nitrates. The idea behind it is to remove proteins and other organic molecules from the water before they are broken down by bacteria and end up as nitrate. Protein skimming on it’s own will not usually remove a problem with nitrates but it certainly helps. Good maintenance of the skimmer is important to keep up the effectiveness of the unit, fatty deposits that build up in the neck of the skimmer cause bubbles to burst more quickly and then the skimmer effectiveness is reduced.

Overstocked and Overfed
The main causes of high nitrates in a reef system along with inadequate filtration media and water flow is too high a biological load. Many people are still guilty of overfeeding their fish and by doing so they are their own cause of nitrate problems. Fish, unlike humans have a quite simple digestive system and they will continue to eat even if their nutritional requirements have already been met, they simply excrete only partially digested food.  Fish are also cold blooded and so gram for gram require far less calories than warm blooded animals to maintain a healthy metabolism and body weight. If you are having problems with nitrate in your system then one of the first things to consider is the amount of food you are putting in. Even though your fish may appear to be hungry you may be feeding them far more than they need. It is quite easy to tell by observing your fish if they are receiving enough food by their weight, most fish should be lean and slender and all too often fish in aquariums can be seen to be considerably overweight. If you are suffering from high nitrates you may want to try cutting down the amount you are feeding them, reduce their feed by 25% and observe your fish over a two week period, depending on the response by the fish you can then adjust the feeding regime accordingly.

Other sources of nitrate in reef aquariums are coral foods and some of these can be quite polluting. The rule of thumb with feeding corals is to feed as much as you can without deteriorating water quality. However if you are suffering from nitrate problems then you already have too high a bioload for the system and liquid coral feeds should be the first thing to cut back on. That having been said dead coral foods are far more polluting than live coral foods. Coral feeding powders, liquid foods and the like can be very polluting without any significant benefit to corals. Similarly any dead phytoplankton added to the aquarium that is not taken up by organisms will quickly decompose and release nitrogenous compounds. Live phytoplankton is less polluting, in fact live phytoplankton will actually reduce nitrates from the water as live phytoplankton will consume nitrates, but bear in mind that if live phytoplankton dies in your aquarium before it is fed on by organisms it too will add to the nitrogenous waste levels.

Another cause of high nitrates may be overstocking, if after considering all other factors you are still having trouble with high nitrates then you may want to reduce your stocking levels. There is no hard and fast rule about stocking levels in reef aquariums, so much depends on the way the system is set up and the filtration methods. The size and species of fish that you are keeping also has a large bearing on the numbers that you can keep. It is worth bearing in mind that larger fish will obviously be more polluting than smaller ones, a single medium sized Tang for example can have the same biological load as twenty small Damsels.

Water Sources
It is important to remember that the source of water for your aquarium needs to be clean. With the low cost of reverse osmosis units these days there are few people that are foolish enough to try to run a reef system without using RO water. There are many things in tap water that cannot be easily tested for and that are detrimental to a reef system, using clean water for evaporation replacement and water changes is essential. If you detect nitrates in your system it is always worth checking that your source water is free of nitrates and that RO membranes do not need replacing.

Lowering Nitrates with Algal Filters
Algal filters are one of the oldest aquarium filtration techniques and they come in a great diversity of techniques. Some are run as refugiums (although a refugium actually refers to a refuge for zooplankton populations), others as algal scrubbers or mud systems. The macro algae that grow in seawater take up nitrogenous products and use them in photsynthesis, hence as they grow they can reduce nitrate levels in the water. There are a number of different macro algae that can be used, Caulerpa species grow particularly well as do Chaetomorpha species, in recent years Chaetomorpha species have been used more frequently as they do not release caulerpins into the water which may inhibit coral growth (although I have never seen any indication of problems with Caulerpa in filters myself).

Many people have algal filters of various designs and yet wonder why they still have problems with nitrates. Often this is because they are not harvesting the algae. Algal filters only work to lower nitrates if you are removing the algae from the water by harvesting it regularly. To enable macro algae to grow fast enough to make it an effective method of lowering nitrates it must have a high intensity light source and a good level of water movement, low light intensity and low water movement will lower the rate of photosynthesis and hence nitrate reduction. Running algal filters with just one or two flourescent tubes is not going to make an effective algal filter, previously I have used 150w metal halides of a low kelvin rating and found these very effective with algal filters. The maximum amount of light that can be used in photosynthesis is around 300µmol·m²·sec, to achieve any where near this with flourescent T5 tubes you are going to need several tubes positioned close together and very close to the surface of the water. When algal filters are running effectively you will be cropping a large amount of algae from them each week or two. When I previously ran algal filters that had an 18 inch square bed size and with a single 150w metal halide I was cropping a quarter to a third of a carrier bag full of Caulerpa every ten days to two weeks. If you are not regularly cropping large amounts of macro algae from your filter then it is not working as an effective means to lower nitrates.

Dosing a Carbon Source and Other Techniques
This is not a new technique and has in fact been around for many years, however it has popped up again recently in the form of dosing vodka to sand beds. Previously sugar or methanol was used and dosed to a carbon denitrator, these were generally delicate pieces of equipment to run and not very popular in the US or the UK. Recently there has been a resurgence of dosing a carbon source to sand bends, this time around in the form of Vodka. This technique works by providing the bacteria with an additional source of carbon which helps to increase their biomass and hence nitrate levels are lowered. The theory behind the technique is reasonable and there has been a lot of anecdotal evidence by reefkeepers that dosing a carbon source has helped them reduce nitrates and improve water quality. Personally I view the method of dosing vodka to reef systems with trepidation due to the inherent dangers it brings, perhaps also because I prefer more natural techniques and have always been able to maintain undetectable nitrate levels with them. If however you do decide to dose a carbon source to your system then you can find some great articles on the web and I recommend that you research the technique carefully.
There are a number of other methods that can be used to reduce nitrates, sulphur denitrators, nitrate absorbing solids and polymers such as Polyfilters will all help to lower nitrate levels, however these can be expensive or fiddly as in the case of the sulphur denitrators. Although these other techniques work in my opinion there are better ways of dealing with nitrates.

Most aquariums that are suffering from high nitrate levels are having problems because the system is not set up or balanced properly. Often this is due to either overfeeding, too high a stocking density of animals or inadequate filtration. Inadequate filtration can be from a number of faults, you may not have enough live rock or water flow in your system, if you are running algal filters then it may be that there is not enough light or water flow to enable rapid growth and harvesting of the algae. Whatever your chosen filtration technique be it Berlin system, deep sand bed, mud systems or algal scrubbers, these are all old and well tested methods of filtration. Provided that these systems are set up correctly and your aquarium is not being overstocked and fed they have the ability to maintain undetectable nitrate levels.