How To Lower Nitrate In Freshwater Aquarium?

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 other aquarium byproducts as ammonia and nitrites, an excessive buildup can be deadly for your fish.

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. 

Still, we have to find ways to manage nitrates. You cannot allow excessive nitrate accumulation in an aquarium as it will eventually affect fish.

Whether you are an expert hobbyist or a beginner, you should be aware of nitrate buildup’s adverse effects and know how to eliminate it. Well, you can never eliminate nitrate buildup, but you can manage it properly. 

In this article, I’ll talk about how to lower nitrate in freshwater aquariums.

The Adverse Effect of Nitrate

Nitrate is not as toxic as ammonia or nitrite. It 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, fish are seen to survive in high nitrate levels. Because they can become gradually accustomed to a beyond healthy nitrate level, 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 nitrate 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 Nitrates in Freshwater Aquarium infographic 2021

How to Lower Nitrate in Freshwater Aquarium with Simple Steps

Before knowing how to lower nitrates in a freshwater aquarium, it is better to get some knowledge on the main reasons for nitrate buildup. If you know the reason beforehand, you can prevent it from forming nitrate in your aquarium. As always, prevention is better than cure. If you can’t stop nitrate buildup, 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.

1) High fish density

Overstocking is the most crucial contributing factor for nitrate formation, which is the root of all causes. If you keep a large number of fish in the aquarium compared to the tank size, nitrate formation will be drastic!

High fish density causes excessive fish waste, and you cannot avoid it anyway. To keep a healthy aquarium, you must prevent overstocking. Instead, the expert aquarists encourage understocking.

2) Overfeeding

Overfeeding is a common cause of nitrate formation, and it is easily controllable. Food leftovers rot to form ammonia and nitrates eventually.

Providing too much food to adorable pets is a common mistake that beginner fishkeepers make. Not only ammonia and nitrates, but they can also cause waste byproducts like phosphates. It will pollute your aquarium in a short period.

3) Accumulated plant materials

Aquarium plants are useful for nitrate control. But at the same time, dead leaves and stems of beneficial plants can bring adverse effects on nitrate buildup.

All biological wastes produce nitrate, so fallen leaves and stems accumulating and rotting at the bottom can form nitrates.

4) Nitrate in the water source

Filling the aquarium with tap water can cause nitrate entrance because sometimes tap water contains high nitrate levels. In the United States, potable water’s nitrate level can be as high as 40 ppm, which is high enough. I suggest keeping the nitrate level below 20 ppm. It should never exceed 30 ppm.

5) Waste accumulation in filter media

Aquarium filters entrap finer biological wastes and keep the water transparent. But the accumulated waste in filter media can still 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.

Prevention for nitrate buildup

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

Prevention for nitrate buildup

1) The right number of fish

Overstocking is deadly! Never jam your aquarium with so many fishes, and never go beyond the actual capacity.

There is a thumb rule for stocking fish in an aquarium. Your fish tank volume in gallons should be equal to the total length of fish in inches. For instance, in a 20-gallon tank, you can keep fish with a cumulative length of 20 inches.

Experts have an interesting say; in your fish tank, keep the only half number of fish that seems ideal to keep! If you do not overcrowd the aquarium, nitrate control will be an easier task for you.

2) Appropriate feeding

Never overfeed your fish. Overfeeding does no good to your fish. Instead, it 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.

3) Nitrate free water source

Before water filling the aquarium, it is preferable to analyze water quality with a testing kit. Testing is straightforward with the kits available in the market; for reliable data, I recommend API Aquarium Test Kit. You should choose water that contains less than ten ppm of nitrates. If your water source exceeds the limit, search for an alternative.

Mitigation for nitrate buildup

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

Mitigation for nitrate buildup

1) Water changes

The easiest way to control excessive nitrate buildup is a regular water change. At least 10% to 20% 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, and in that case, you can use a water purifier or RO water. Regular water changes with fresh water will keep the nitrate level sufficiently low. In most cases, you will not require any other mitigation.  

2) 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 nitrate-absorbing filter media and use it in your existing filtration system. In my experience, it will do quite well. 

3) Plants to reduce nitrate

Aquatic plants can absorb ammonia, nitrites, and nitrates. So converting your fish tank to a planted aquarium is the natural way to control nitrate formation. 

Having live plants in the aquarium is an exciting idea. They not only denitrify but also beautify your aquarium gorgeously. There are many plants available beneficial for nitrate control

Mangrove plants are popular among aquarists, which are suitable both for freshwater and saltwater aquariums. But plants can only use a limited amount of wastes, and dead plant matter can also be a nitrate source.

So, if you go for a planted tank, carefully select the fish density and ensure regular cleaning. For your planting guide, check our articles on how to plant aquarium plants

4) Regular replacement of filter media

Replacing the filter media is routine maintenance that you need to perform. It is very vital to lower the dissolved nitrate level.

Filter media entraps waste to make the water clear, but those biological wastes can be nitrate formation sources.

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.

5) Cleaning the substrate

Only water changes will not be sufficient over a long period. A rigorous aquarium cleaning (including the substrate) 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.

But the substrate houses beneficial bacteria. A thorough cleaning can destroy their colony. Don’t replace all the water during cleaning, and don’t take the substrate material out of the tank.  

6) Chemical treatment

Chemical treatment all alone is not sufficient for nitrate control. Still, it can help. There are some reliable water purifiers and nitrate removers available in the market. I recommend Fluval Hagen Biological Cleaner for Aquariums

When dosing chemicals in the fish tank, I’ll suggest following the manufacturer’s instructions in the letter.


I will suggest keeping the nitrate level below 20 ppm in freshwater aquariums. In this condition, your fish get the best comfort and health.

Nitrate is colorless and odorless, and its presence is difficult to notice. Regular testing is the only way to monitor nitrate levels. Nitrate test kits are available in the market.

You will have to give some labor for nitrate control, but you will be beneficial in many ways in return.

Healthy and energetic fish are comfortable to keep avoiding associated complicity. They are potent in preventing diseases. Every fishkeeper loves to see your aquatic mates are happy, brisk, and joyful!

Sujit Modak

Leave a Comment