Managing nitrate levels in a fish tank is an ongoing task that all fishkeepers share. Important for maintaining a stable ecosystem in which your plants, corals or fish can thrive, testing for high nitrate levels, and lowering them if they spike above safe bounds, should be a natural part of your regular care routine. If you’ve recently started your first aquarium, or are struggling to keep nitrate levels down, join our experts as they break down everything you need to know about what nitrates are, what causes their levels to rise, and what to do when they do.
What is nitrate?
Nitrate (NO3) is a simple compound made up of nitrogen and oxygen which is naturally found in aquarium water. This is because nitrate is produced during the last phase of the nitrogen cycle. If you’re unfamiliar with the fish tank nitrogen cycle, you can learn all about it (including how to complete your first cycle safely in a brand new tank) in our full beginner's guide. We’ve summarised the main three stages of the cycle below in a quick review:
- Ammonia (NH4) is produced. Ammonia is produced through the decomposition of protein (in this case fish waste and other organic compounds like uneaten food). Ammonia is toxic to aquatic life, particularly in high levels.
- Nitrite (NO2) is produced. Ammonia is converted into nitrite by a nitrifying bacteria called nitrosomonas. Nitrites are also toxic to your tank inhabitants, but this stage is necessary for the last phase of the cycle to take place.
- Nitrate (NO3) is produced. Nitrite is converted into nitrate by another beneficial bacteria called nitrobacter. Nitrate is the least toxic of the three nitrogen compounds, and is the easiest to manage and keep at safe levels.
What causes high nitrate levels in fish tanks?
As nitrates are created in the last phase of the nitrogen cycle, we can find the cause of high nitrate levels in a tank by backtracking through the cycle to the first phase: ammonia production. After all, the volume of nitrate is directly proportional to the amount of ammonia created during the first stage of the cycle, so if more ammonia is created, nitrate levels will also rise.
Ammonia itself is caused by a few different factors, with the most common being the decomposition of organic matter in the water such as excess fish food from overfeeding, fish waste from overstocked tanks, and plant debris from poor pruning and cleaning practices.
How to check nitrate levels in a fish tank?
The easiest way to get an accurate reading of the nitrate levels in your fish tank is to use a nitrate water test kit. These work by taking a small sample of your tank water and combining it with a nitrate test solution. After a few minutes, the colour of the water and solution mix will change, and the results of the test can be ready by comparing the solution colour against the included colour card.
Which test you should use will depend on the type of tank you have, and each individual brand will come with specific instructions for use. To make sure you get accurate results every time, we recommend using any of the following test kits, depending on your tank type.
Salifert Nitrate Profi Test Kit, £11.75 - for reef tanks
NT Labs Test Kits - Nitrate, £8.70 - for freshwater tanks
API Nitrate Test Kit, £12.80 - for fresh and saltwater tanks
What is a safe nitrate level for a fish tank?
Nitrate levels in aquarium water are read in parts per million (ppm), which is equivalent to milligrams per litre (mg/L). In nature, nitrate levels are usually very low, measuring below 5ppm, but this isn’t often the case for aquariums. The levels which are considered ‘safe’ change depending on the type of aquarium you have and the tank inhabitants you keep, with the main tank types detailed below:
- Freshwater tanks should be kept below 50ppm, but to be on the safe side (and to account for more sensitive fish species) we recommend aiming for nitrate levels below 25ppm.
- Saltwater tanks can be sensitive to high nitrates, with recommended levels sitting below 20ppm. Some marine species also require a lower level than this, so make sure to check the individual needs of your fish.
- Reef tanks require a particularly low level of nitrates in order for the corals to thrive. For these, we recommend that soft and large polyp stony (LPS) coral tanks be kept between 1-5ppm, while small polyp stony (SPS) corals prefer even lower levels of below 1ppm.
- Breeding tanks are a special case, with recommended nitrate levels of lower than 20ppm.
What level of nitrate is toxic to fish?
While toxicity to fish varies depending on a range of elements from tank type to fish species hardiness, for the most part you’ll begin to see negative health effects from nitrates in your tank inhabitants when levels reach 100ppm.
How do fish act when nitrates are high?
Nitrate poisoning in fish presents itself in a wide variety of symptoms, with most signs being similar to that of stress. This includes loss of appetite, rapid breathing (shown through faster than normal gill movements), laziness and slow reaction times, and lying on the bottom of the tank. There can also be long term effects of high nitrates on fish, with stunted growth and susceptibility to disease being the most common.
How do I lower the nitrate levels in my fish tank?
Now that we’ve given some background on the causes of high nitrates and how to test your water to find out the concentration of this compound in your tank, the next thing you need to learn is how to lower nitrate levels when they go past the recommended boundaries.
There are a couple of different techniques to use to bring nitrate levels down, all of which we’ve detailed below in an easy guide:
- Perform a water change. Water changes are the easiest way to manage high nitrate levels in your tank. Not only can a water change very quickly drop the concentration of nitrates in your tank when they spike, but regular water changes (every 2 to 4 weeks) are also a key part of maintaining a low level consistently.
- To perform a water change in your tank, all you need to do is remove a percentage of the water from your tank (around 25%) and replace it with new water. The new water should either be dechlorinated tap water (use a dechlorinator like Aqua One’s Water Conditioner to clear water of harmful chemicals) or reverse osmosis (RO) water. No matter which type of water you use, make sure to test it for nitrate levels before making the change. If the new water is also high in nitrate, this method won’t work to lower the levels.
- Add nitrate-removing liquid. Some fishkeepers who struggle to keep nitrate levels low can rely on aquarium treatments like Tetra Nitrate Minus. Treatment instructions will change by product, but most work through weekly doses to remove nitrates, making them easy to add into your regular aquarium care routine.
- Add ammonia-removing media to your filter. Products like Fluval’s Zeo Carb can be added into your aquarium filter to target and lower ammonia levels before they’re converted by the nitrifying bacteria into NO2 and NO3. These need to be replaced around once a month, but are a great way to manage levels in the long term.
- Improve feeding habits. While not an instant fix, one thing you can do to encourage low nitrate levels in the long term is to improve your fish feeding habits. Nitrate levels spike when excess waste materials decompose in the water and create too much ammonia, so being careful not to overstock your tank with fish, or feed them too much, will help avoid these problems.
What is the fastest way to lower nitrates in an aquarium?
If you’re looking for ways to reduce nitrate levels in an aquarium fast, your best course of action is to perform a bigger water change than normal. As long as you take careful steps to make sure the new water is the right temperature, pH and has low nitrate levels, you can perform a water change of up to 50% without risk of shocking your fish.
How to lower nitrates in fish tank naturally
While performing water changes and dosing your tank with nitrate-removing liquid will help to keep levels low, the best way to achieve low nitrate levels naturally is to add live aquatic plants to your tank. Aquarium plants are natural nitrate filters, absorbing nitrates from the water to use them as fertiliser, while simultaneously creating a beautiful tank for you and your fish to enjoy.
To learn more about caring for aquarium plants as a beginner, read through our dedicated planted aquarium guide, or browse through our collection of live aquarium plants to get started building your own.
Keep nitrate levels low in your aquarium with Aquacadabra
From water quality tests and dechlorinators to nitrate-reducing treatments and live aquarium plants, we have everything you need to keep your fish tank’s nitrate levels within safe bounds. If you need any additional help or personalised guidance, get in touch with our expert team who can advise on how to keep your aquatic ecosystem happy and healthy.