Water testing series

part 3 - Nitrate (NO3)

Nitrate - The end of the road... or is it?

In this series so far we've discussed Ammonia & Nitrite, and how to prevent/treat spikes associated with either.  Nitrate is the next parameter that you will find after ammonia and nitrite. It is the last nitrogenous (nitrogen based waste product) that you will test for. It is however, not the end of the road! Contrary to popular belief, it is NOT the end of the biological filtration cycle, but we'll get to that bit later on in this guide. Firstly we need to explain what nitrate is and why it matters.

Ammonia > Nitrite > Nitrate

As previously discussed in the Ammonia guide, The nitrogen cycle (de-nitrogen cycle) is the processes of how Ammonia is converted to Nitrite by beneficial bacteria, followed by Nitrite being converted to Nitrate by a different set of beneficial bacteria. Both compounds sound incredibly similar, and this leads to a lot of confusion when talking about parameters. Both of these chemicals are very different and interact in aquariums differently.

In order to be able to understand and treat nitrate, it's important to understand ammonia & nitrite. Separate guides for these two parameters can be found below as part of our water testing series.

Nitrate (NO3 - Nitrogen + 3 Oxygen)

What is nitrate?

Nitrate is a nitrogenous compound that is formed by the oxidation of Nitrite in certain filter bacteria (nitrobacter, nitrospira among others) it can also be present in tap water and can be released by other organic sources (fish food, decaying matter). It is considered the end product of aerobic biological filtration.

Why does it matter?

Nitrate is considerably less deadly when compared to ammonia and nitrite, however it does present its own problems. High levels of nitrates can lead to weakened immune systems and reduced or stunted growth. A more apparent problem associated with high levels of nitrate is algae growth.

Level of Nitrate ppm (Parts Per Million)


No nitrate detected by test kits. The best level to keep all fish and inverts (especially delicate species), although it may not be possible to maintain.


Values below this are ok (lower the better). Levels above this might lead to health problems. Algae growth might become a problem.


Most Fish will start to show signs of nitrate poisoning at this level. Deformed/Stunded growth, stress behaviour and problems with breeding & raising young might occur.


Extremely high! Fatalities are VERY common. Disease caused by stress very likely. Agae growth might be prolific.

Symptoms of high nitrates

Because nitrate is less deadly that ammonia and nitrite, it is quite often seen as less of a problem, but this is not necessarily true. High levels of nitrate can lead to increased stress in fish, leading to a higher likelihood of disease, such as open sores, and infected tissue.

At high enough levels, if can kill fish (especially more delicate species of fish). Long term symptoms of nitrate poisoning range from deformed/stunted growth to lethargic behaviour.

How to prevent & treat a nitrate problem?

Similar to nitrite spikes, one of the main causes of a nitrate spike can be a spike in the previous compounds (Ammonia & Nitrite). Because of this, one of the best ways to prevent nitrate spikes is to follow the prevention techniques to reduce ammonia.

Having said that, another major cause of high nitrates can be from your water source. Tap water can contain nitrates and may fluctuate over time. This is problematic and is a slightly trickier problem to fix, although not impossible.

More prevention tips can be found in the Ammonia guide.

Here are some other ways you can treat a Nitrate spike.

Tap water alternatives (Aquarium only)

If you have nitrates in your tap water then it may be problematic to keep doing water changes with tap water. RO (Reverse osmosis) water is essentially normal water that has been filtered to the point where you are left with pure water (in theory, this depends on setup and efficiency). Because this water is devoid of mineral content, as well as other pollutants such as nitrates and phosphates, it can be an ideal option to perform water changes with.

Keep in mind that fish do need a certain level of mineral content (differs between species) and a preferred level of hardness (gH & kH - leading to changes in pH), so you should only use RO partially with tap water, or be prepared to re-mineralise RO to reach your decided upon parameters.

Extra info - Rainwater.
It's not advised to use rainwater due to possible contaminates. Contaminates can be from the rain falling through pollution (car fumes, factory fumes & chimney fumes) and from how the rain is collected (roof surfaces, guttering & storage vessels.) This is also one of the reasons that pond keepers need to keep up with regular water changes.

Use biological enhancers - Encourage anaerobic filtration

The conversion of ammonia to nitrite, and nitrite to nitrate by means of bacteria, can be classified as aerobic filtration. This mean that the bacteria which are responsible for this conversion need oxygen rich water in order to survive. Internal and external filters are ideal places for these bacteria, as the flow of oxygen rich water through the media provides the perfect environment for them to thrive.

The bacteria that can convert nitrate to free nitrogen however, prefer a low oxygen environment. This is because they obtain their oxygen source from the nitrate compound itself. This form of biological filtration is referred to as anaerobic filtration. Anaerobic filtration is harder to establish than aerobic filtration and can take much longer to work efficiently enough to see results. The best way to encourage anaerobic filtration is to implement low oxygen zones within your filter/secondary filter.

This can be done by using specialised filter media that provides an anaerobic environment within the media, and implementing slow flow filtration so as not to force aerated water through the media too much. It’s worth noting that it’s not recommended to change your filter to accommodate only anaerobic filtration, as this might deter or even stop aerobic filtration!

Use Plants

Aquatic plants need to take their form of nitrogen from either the water or substrate (Plant soil, root tabs etc).  Plants are amazing filters for removing nitrogen-based compounds from the water. If you’re not looking to create a dense planted tank, and prefer to use plants for function, then look for fast growing plants and ones which grow above the surface as these will be more efficient.

Remember, plants need more than just nitrogen! If you're going to use plants to help remove nitrate, then make sure you dose your aquarium with a good comprehensive fertilliser - one without nitrogen (you've already got that!).

Extra info - Fertilising nitrates

In moderate - high density planted aquariums or those with low fish stock, you may need to dose nitrates (ammonium, urea or nitrates might be used). Before you start dosing you need to measure what levels of nitrate (and ammonium) you already have. You don't want to make any problem you already have worse!