How to Lower Nitrate in Freshwater Aquarium

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If you are keeping fish or other aquatic pets in an aquarium, you cannot avoid nitrate buildup, and nitrate originates from fish waste, food leftover, or rotten plant materials. Nitrate is odorless and colorless, thus challenging to notice its presence. Though nitrate is not much less toxic than ammonia, an excessive buildup can be deadly for your fish.

Whether you are an expert hobbyist or a beginner, you should be aware of the adverse effect of nitrate buildup and know how to get rid of it. Well, you can never eliminate nitrate buildup, but you can manage it properly. In this article, we’ll talk about how to lower nitrate in freshwater aquarium.

Both ammonia and nitrate are byproducts of aquarium waste, and more specifically, nitrate comes from ammonia after a multistage chemical reaction. The toxicity of ammonia is acute, and therefore it is crucial to establish a nitrogen cycle in an aquarium. In a nitrogen cycle, ammonia converts into nitrite first, and then nitrite converts into nitrate through an oxidation rotation.

This biological cycle is essential to maintain a healthy environment for aquarium inhabitants, and beneficial bacteria promote this cycle. While producing nitrate from ammonia is our primary desire because nitrate is much less toxic, we still have to find ways to manage nitrates. We cannot allow excessive nitrate accumulation in an aquarium as it will eventually affect fish.

The adverse effect of nitrate

Nitrate is not as toxic as ammonia or nitrite and does not show any immediate effect, but in a long time exposure, it can be deadly as well. Nitrates reduce the immunity of fish and make them prone to diseases. Fishes in a high nitrate aquarium, do not grow satisfactorily.

A highly concentrated level of over 100 ppm can stress your fish severely. They lose their health, become lethargic and red blotches can appear on their skin. It is not unusual to see unhealthy fish dying suddenly.

Sometimes, at a high nitrate level, fish can become gradually accustomed, but they will not be lively and spontaneous. If you add new fish in such an unhealthy aquarium with an elevated nitrate level, they may not survive.

High nitrates levels are harmful to live plants. Although plants can utilize nitrates, excessive nitrates will boom algae growth, which will cause asphyxiation (lack of oxygen) for the plants.

How to lower nitrate in freshwater aquarium with simple steps

Before knowing how to lower nitrates in freshwater aquarium, it is better to get some knowledge on the main reasons for nitrate build-up. If you know the reason beforehand, you can take action to prevent it to form in your aquarium. As always, prevention is better than cure. If you can’t stop nitrate build-up then you need to go for mitigation. 

Factors promoting nitrate buildup

To deal with nitrate buildup, we have to know the primary causes that contribute to nitrate formation. Because to fight well, you have to know your enemy well. The primary nitrate source is ammonia formation in an aquarium, and ammonia comes from all sorts of biological wastes associated with the aquarium.

High fish density:

Overstocking is the most important contributing factor for nitrate formation. If you keep a large number of fish in the aquarium compared to the tank size, nitrate formation will boom! High fish density causes excessive fish-waste, and you cannot avoid it anyways. To keep a healthy aquarium, you must prevent overstocking. Instead, the expert aquarists encourage understocking.


Overfeeding is the most common cause of nitrate formation, and it is easily controllable. Food leftovers rot to form ammonia and nitrates eventually. Providing too much food to the adorable pets is a common mistake that beginner fishkeepers do. Not only ammonia and nitrates, but it can also cause waste byproducts like phosphates. It will pollute your aquarium in a short period.

Accumulated plant materials:

There are many aquatic plants useful for nitrate control. But at the same time, dead leaves and stems of beneficial plants can bring adverse effects on nitrate control. All biological wastes produce nitrate, so, fallen leaves and stems accumulating and rotting at the bottom can form nitrates.

Water source:

If you fill the aquarium with tap water, it can be another source of nitrate entrance, because sometimes tap water contains high nitrate level. In the United States, the nitrate level in potable water can be as high as 40 ppm, which is high enough, while we are targeting to keep nitrate level in an aquarium is 50 ppm maximum.

Waste accumulation in filter media:

Aquarium filters entrap finer biological wastes and keep the water transparent. But the accumulated waste in filter media still can be a source of nitrate formation, because they are out of sight but present in the water. This one is a nitrate source that people usually forget; irregularity in filter cleaning can result in a high nitrate level.




There are some mitigative controls available, but at first, we will focus on controlling the nitrate source.

The right number of fish:

Overstocking is deadly! Never jam your aquarium with so many fishes, never go beyond the actual capacity. There is an interesting thumb rule for the right number of fish in an aquarium, keep only half that seems ideal to keep! If you do not overcrowd the aquarium, then nitrate control will be an easier task for you.

Appropriate feeding:

Never overfeed your fish. Overfeeding does no good rather can be very harmful to the aquarium environment. Feed only as much as your fish can finish in just two minutes. Try to minimize food leftovers as much as possible.

Nitrate free water source:

Before water filling to the aquarium, it is preferable to analyze water quality with a testing kit. You should choose water that contains less than ten ppm of nitrates. If your water source exceeds the limit, search for an alternative.


No matter how reasonable your preventive control is, you have to take some mitigation for nitrate buildup. 

Water changes:  

The easiest way to control excessive nitrate buildup is a regular water change. At least 30% to 50% of the water should be changed in a weekly schedule. But the replacement water must be nitrate-free and suitable with other parameters.

Sometimes tap water contains a high level of nitrates; in that case, you can use a water purifier or RO water. Regular water changing with fresh water will keep the nitrate level sufficiently low, and in most cases, you will not require any other mitigation.  

Thorough cleaning of the aquarium: 

Only water changes will not be sufficient over a long period, and a rigorous aquarium cleaning is essential at least every three months. A significant amount of wastes can be entrapped in the substrate, and only a comprehensive cleaning can remove those.  

Nitrate absorbing filters:

If your aquarium suffers from severe nitrate buildup and water changes not coming as enough, you can use special filters that absorb nitrate. There are anaerobic denitrifying biofilters available in the market, and they will reduce the dissolved nitrate level in the water.

Special filters called denitrators are useable for nitrate lowering purposes, but such devices are expensive. Instead, you can buy a nitrate absorbing filter media and use it in your existing filtration system, and it will do quite good. 

Plants to reduce nitrate: 

Aquatic plants can absorb ammonia, nitrites, and nitrates, so a planted aquarium is a natural way to control nitrate formation. Having live plants in the aquarium is an exciting idea, not only for denitrification but also for beautifying your aquarium gorgeously. There are many plants available, beneficial for nitrate control.

Mangrove plants are popular among aquarists, which are valid both for freshwater and saltwater aquariums. But plants can only use a limited amount of wastes, and dead plant matters can also be a source of nitrate. So, if you go for a planted tank, carefully select the fish density and ensure regular cleaning. 

Regular replacement of filter media: 

Replacing the filter media is routine maintenance that you need to perform, and it is essential to lower the dissolved nitrate level. Filter media entraps waste to make the water clear, but those biological wastes can be sources of nitrate formation. The best practice is to replace the filter media weekly, but if it becomes a bit tough for you, set 10 days frequency, but not more than that. 


We will suggest keeping the nitrate level below 50 ppm for your freshwater aquarium. As nitrate is colorless and odorless, its presence is difficult to notice. Regular testing is the only way to monitor nitrate levels, and test kits are available in the market.

You will have to give some labor for nitrate control, but in return, you will be beneficial in many ways. Healthy and energetic fish are comfortable to keep avoiding associated complicity. They are susceptible to preventing disease and brisk. Every fishkeeper loves to see your aquatic mates are happy and joyful!

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Sujit Modak Aquarium Tales Owner
Sujit Modak

Hello, everyone! Welcome to my aquarium blog. Fishkeeping is my passion, and I started this fascinating hobby back in 2006. Besides my engineering profession, I deeply studied many fishkeeping topics since I started building my home aquarium. I researched effective aquarium filtration and lighting of planted aquariums. I am keeping 20+ species of freshwater and saltwater fish as my aquatic pet collection. I successfully experimented with a complex ecosystem inside the aquarium, biotope aquariums, aquaponics, etc. I would love to share some learnings from my hands-on experience of the last 14 years. Hopefully, my sharing will be somewhat helpful to make your aquarium journey awesome! 

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