High Nitrates in Planted Tank – Causes & Fixes

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If you know anything about the aquarium world, you already know about the 3 horsemen: ammonia, nitrites, and nitrates. You also know that ammonia and nitrites are potentially deadly in any concentration above 0 and that nitrates are not.

Ammonia can come from various sources, such as fish waste, uneaten fish food, and organic matter decomposing in the tank.

This isn’t necessarily a problem in a well-cycled environment since the Nitrosomonas bacteria will convert ammonia into nitrites. Then, nitrites are converted into nitrates by the Nitrobacter bacteria.

This cycle keeps the setup stable in theory. Except there’s a catch. There are no bacteria to consume or convert nitrates into something else.

And while nitrates aren’t as dangerous as the previous 2 chemicals, they can still pose serious health risks to your aquatic life in a high-enough concentration.

This is exactly what we’ll be discussing today.

How High Should Nitrates Be in a Planted Tank?

Since nitrates get more toxic as their concentration grows, the lower they are, the safer the environment. While some suggest that a concentration of up to 20 ppm is safe for your aquatic life, I disagree.

Such a concentration may seem safe at first, but it will create health problems for your fish and invertebrates over time.

To prevent that, always look to keep the nitrate levels below 10 ppm. Preferably below 5 ppm. Interestingly enough, heavily planted tanks display lower nitrate levels than those with fewer live plants. Even more, interestingly, that’s not always the case. But why?

Why is Planted Aquarium Nitrate Level Very High?

Just because you have a planted tank doesn’t mean that the nitrate levels are low or within the acceptable range.

There are several causes for high nitrates in a planted fish tank, including:

  • Overcrowding – Having too many fish crowded in a too-small space is bound to spell environmental disaster. The fish won’t only be overly stressed and aggressive due to territorial fights and food competition, but they will also produce more waste than the system can process. The accumulated waste will overburden the tank’s microfilm, causing the bacteria to work overtime. Nitrates will accumulate at an accelerated rate, faster than you can dissipate them via water changes. Which is clearly not ideal.
  • Overfeeding – Feeding your fish more than they can eat is almost always a bad idea. Not only do they risk developing digestive problems because of that, but the residual food will also decay on the substrate and contribute to a higher and faster accumulation of nitrates. Always feed your fish what they can eat in one serving and try to remove any residues immediately if possible.
  • Lack of tank maintenance – The smaller the tank is, the higher the need for regular maintenance. The lack of maintenance will contribute to a faster accumulation of ammonia and nitrites and, consequently, nitrates. The frequency of maintenance generally depends on your tank’s size and the fish species you’re housing, and how dirty they are.
  • Lack of water changes – While water changes also fall into the maintenance category, they are a special cleaning measure of their own. Water changes are necessary to reoxygenate the water, clean the environment, and, most importantly, dilute the nitrates. As a matter of fact, water changes are the primary method of controlling nitrate levels. Their frequency will vary depending on the tank’s size, how messy the fish are, what you’re feeding the fish and how often, and many other factors. Generally, you probably need to perform a water change weekly to keep the environment stable and healthy.
  • A clogged or polluted filter media – The tank’s filter media is the main defensive line against ammonia and nitrites. The problem is when the filter becomes clogged with dirt and gunk and no longer performs as it should. To prevent this issue, I recommend cleaning the filter regularly, always with dechlorinated water, to prevent chemical poisoning.
  • Improper water changesTap water is especially dangerous in this sense. Even if you’ve dechlorinated the water before use (which is a must), you still run into the danger of increasing the nitrate levels in the tank considerably. That’s because the water sterilization system isn’t perfect, leading to tap water containing levels of nitrates up to 44 ppm. These may be safe for humans, but they will bring death to your fish tank.

Sure, having live plants will mitigate the problem to some degree. But, as you can see, the situation is sometimes too much for your plants to fix it.

Not to mention, live plants can actually contribute to the problem, as we will soon see.

Can Plants Cause High Nitrate Levels in Aquarium?

Yes, live plants can cause high nitrate levels in the aquarium, but not the way you think. Plants don’t have a mechanism by which they produce nitrates. Instead, nitrates are the biological result of dying plants flooding the environment with decaying organic matter.

This isn’t a problem with the plants themselves but rather with the lack of proper tank maintenance.

A heavily planted aquarium is especially at risk since there are a lot of plants capable of producing more dead matter than your environment can handle.

The real danger here is having dead leaves being sheltered by the live ones, so you won’t be able to detect them in time. The decaying leaves will feed the masses of Nitrosomonas and Nitrobacter, resulting in more nitrates than you’d need.

The only way to prevent or eliminate the issue is by monitoring your aquatic setup regularly and cleaning any plant-related residues in time.

So, this is more of a maintenance problem than a live plant one.

What Happens if Nitrate Levels are Too High?

High-enough nitrate levels will affect your fish to the point of killing them. This can happen over time or suddenly, depending on the concentration of nitrates and how abruptly the nitrate levels increase.

In this sense, we have 2 main aspects to discuss:

  • Nitrate poisoning – This generally takes place over time since it results from prolonged exposure to higher-than-normal nitrate levels. Your fish will experience specific physiological symptoms, which can begin as soon as several hours after exposure to the unsafe nitrate levels. Interestingly enough, some fish species are more sensitive to nitrates than others depending on water requirements (saltwater or freshwater), age, health status, etc.
  • Nitrate shock – Nitrate shock occurs when the fish are exposed to a sudden and massive increase in nitrate. Nitrate shock can kill the fish within 24 hours or less, depending on the concentration.

Some of the main symptoms of nitrate poisoning in fish include:

  • Loss of appetite
  • Accelerated breathing rates and breathing difficulties
  • Erratic swimming and balancing problems
  • Hiding behavior and laying on the substrate
  • Visible lethargy and disorientation

Fortunately, most cases of nitrate poisoning are reversible, provided your fish get immediate attention and treatment.

How to Lower Nitrate in Fish Tank Naturally?

There are 2 aspects worth discussing here: how to manage the high levels of nitrates and how to prevent them.

So, let’s take them each at a time.

Lowering Nitrate Levels

If you’ve detected nitrate levels above the safety threshold, consider the following:

  • Water changes – You may need to perform a larger water change to dilute the nitrates and restore the environment’s stability. If your fish don’t cope too well with massive water changes, consider moving them into a temporary setting during the procedure.
  • A nitrate filter – Nitrate filters are ideal for not only removing nitrate but other water contaminants as well. This approach is often necessary if nitrate pollution is severe and in need of urgent correction.

While these are good solutions to your nitrate problem, prevention is an even better one.

Preventing High Nitrate Levels

The prevention mechanism relies on several strategies, such as:

  • Avoid overcrowding – This pretty much goes without saying. Keep your fish within acceptable number parameters to prevent overcrowding, and nitrate levels are far less likely to spike.
  • Avoid overfeeding – Only provide enough food for your fish to consume within 1-2 minutes. Anything above is excess and will eventually harm the environment and the fish.
  • More plants – A heavily planted environment will have naturally lower levels of nitrates than barebones ones. That’s because plants use nitrates daily to support their physiological processes. Some plants are so effective at consuming nitrates that nitrate fertilizers may be necessary to support the plants’ increased ‘appetite.’
  • Regular maintenance and water changes – These are the best tools in your nitrate-prevention toolbox. Remove dead matter, fish waste, and food leftovers, and perform weekly water changes to keep the aquatic environment clean and fresh, and your fish will thank you for it.
  • Clean the filtration system – The filter also needs regular cleaning, depending on its workload, filter type, tank size, fish density, waste-producing capabilities, etc. Just make sure you don’t use any chemicals to clean the system, avoid tap water due to its chlorine content, and don’t clean it too thoroughly or too frequently. Doing so may eliminate the biological media inhabiting the filter, leading to ammonia spikes soon.

But can you use specific plants to lower nitrate levels naturally in your fish tank?

Best Aquarium Plants to Remove Nitrates

As a general rule, fast-growing plants are considerably more effective at removing nitrates than slow-growing ones.

With that said, here are some names worth mentioning on this list:

  • Water Wisteria – This flowering plant doesn’t require CO2 supplementation, is easy to keep, and grows fast. It’s one of the best, if not the best, the choice for nitrate reduction and nitrate control due to its high nitrate consumption rate. It’s also great for providing your fish with hiding areas and contributing to a lusher environment. The problem is that water wisteria grows fast and can cut the fish’s access to the water’s surface. It can also prevent sunlight from reaching other plants and aquatic life. So, regular trimming is necessary to prevent that.
  • Moneywort – You may know this one as Bacopa monnieri. This plant can grow up to 30 inches or more and can quickly invade and overwhelm the entire aquatic setup relatively fast. It doesn’t grow as fast as water wisteria, but it still needs trimming from time to time. Moneywort requires higher levels of light, especially since this is a flowering plant. The higher light levels come with the risk of algae bloom, so always keep an eye on that if you have Moneywort.
  • Duckweed – This is a special entry due to duckweed’s ease of care and increased popularity. This surface-floating plant will provide your aquarium with a unique look and vibe. Duckweed is great for nitrate control and as an algae-deterrent since the plant will cover the entire water surface, cutting the algae’s access to sunlight. The problem is that duckweed grows fast and can prevent submerged plants from getting adequate sunlight. So, you should find a way to work around that.
  • Brazilian pennywort – This versatile and resilient plant comes with low requirements and an increased appetite for nitrates. This plant can grow rooted or floating and is excellent at controlling the algae population. Just beware of the plant’s growth rate. Brazilian pennywort doesn’t have a maximum size since it will grow indefinitely until trimmed.

There are a variety of other tank plants that you can use to control nitrates, such as frogbit, dwarf sag, and many others.

Naturally, you need to use live plants in addition to other nitrate prevention mechanisms for the best results.


Nitrate is a natural byproduct of any aquatic setup with an active biofilm. While nitrate isn’t as toxic as ammonia, nitrites, and other components, it’s still dangerous in high-enough concentrations. The main problem is that nitrate naturally is notoriously difficult to remove from the environment.

Plants are your best option, but that’s about it. So, you need to improvise.

Change the water regularly, maintain a healthy maintenance routine, clean the filter media, and constantly monitor nitrate levels.

I also hope that this article can help you in this sense.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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