Controlling Nitrates

Controlling nitrates in your aquarium

By Dave Hulse, Technical Consultant at Tetra


High nitrate levels are a problem all fishkeepers will encounter at some point, whether that’s dealing with the problem directly or treating one of the biggest knock-on effects of high nitrates – algae! Nitrate and phosphate levels are normally quite low in most aquariums, helping to keep the growth of algae at bay. However, if conditions rise then algae will take advantage and their population will bloom, which can make your tank look really unattractive.

Traditionally we thought of nitrate as non-toxic, certainly to adult freshwater fish, and certainly at concentrations likely to be encountered in the aquarium or fish farm. This assumption is starting to be questioned as our tools for researching fish physiology, behaviour and overall welfare become more advanced.

 Where does nitrate come from?

When thinking about nitrate control in the aquarium it helps to think first of the inputs and the outputs. There are two sources of nitrates in the aquarium, the waste from your fish and the ‘ambient’ nitrate level - the concentration in your tapwater. Fishkeepers living in upland rural areas will probably be lucky enough to have concentrations of nitrate in their tapwater so low that they would be undetectable with standard aquarium testing kits. However, many parts of the country are described as ‘nitrate vulnerable zones’ where the inputs of nitrogenous fertilisers and animal manure on the land is controlled to prevent further elevation of an already high nitrate level in the water supply.

In the aquarium, nitrate from fish and plant wastes will be primarily effected by the quality of the fish food. A typical tropical fish flake food, such as TetraMin, may have a protein concentration of around 45%. Typically, we would expect 16% of this protein to be nitrogen, and therefore for every 1 gram of food added there could be up to 72 mg of nitrogen released which should be rapidly cycled from ammonia to nitrate assuming there is a healthy biofilter. This is where food quality becomes important. Protein is used by the fish for growth and the repair or tissues, but also for energy production in the absence of sufficient oil in the diet. A poorly formulated diet will fail to spare protein for growth leading to higher ammonia production. Using poorly digested protein sources will lead to more protein in the faeces which is destined to be broken down to ammonia and on into nitrate.

Another way that nitrate may leave the aquarium is through bacterial denitrification. The nitrogen cycle sees bacteria that live in oxygen-free environments using nitrate as an oxygen source (nitrate is N and 3 oxygens: NO3-). These oxygen free pockets can be found deep in the aquarium gravel. Products such as Tetra EasyBalance rely on these bacteria to lower the nitrate level of the aquarium. In a large scale trial using a range of home aquariums, the average nitrate level dropped from 40 mg/l to below 20 mg/l over a mere 8 weeks. In laboratory trials over a 24-week period, EasyBalance maintained a nitrate level of around 10 mg/l when the level in the untreated control tanks soared above 180 mg/l.

Helping to prevent to outputs

So, how can we properly treat and decrease these outputs to stop the build-up of nitrate in our water? Sticking with the fishes’ faeces first, a tank that is kept clean with a gravel hoover and filter sponge cleaning will remove faeces from the water before it can fill the tank with more nitrate. Plant decay in the water will also release nitrates from the proteins within their tissues. Proper tank hygiene and removal of decaying plants will remove protein before it can become nitrate.

The best way to keep nitrate build-up at bay is via a partial water change. Just think, ‘the solution to pollution is dilution’! However, it is not simply a case of a 20% water change leading to a 20% reduction in the nitrate concentration. We have to also factor the ambient nitrate level in. In many instances, fishkeepers find that their large partial water change is having no effect on the nitrate level according to their testing kit. At high concentrations, it can be quite tricky to see a difference in 75 and 100 mg/l on a colourimetric nitrate testing kit. Couple this with a high ambient nitrate concentration and the actual drop in nitrate concentration following a 25% partial water change with tapwater whose nitrate concentration is 25 mg/l, may be from 110 to 80 mg/l. This is a considerable decline but is hard to actually detect via a simple colourimetric nitrate testing kit.

Ambient nitrate levels in your tap water

The ambient tapwater nitrate level can really hinder nitrate control via water changes in the freshwater aquarium, so one could go down the route of tapwater purification via reverse osmosis or deionisation. However, if we explore other outputs of nitrate from the tank we can recruit simpler, cheaper alternatives. As mentioned above, the principal reason we want to manage the nitrate level in the tank is because, along with phosphate, it is likely to lead to algae problems. All plants need nitrate and phosphate as macronutrients, so why not grow live plants in the aquarium to outcompete the algae? For many fishkeepers, a fully planted tank, with substrates, high intensity lighting and CO2 supplementation is a challenge too far, however there may still be low-light-loving plants that do not require a substrate that could be suited to your tank, such as Java fern.