What Causes High Nitrates In Aquariums?

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Detecting high levels of nitrates in a fish tank can be pretty frustrating. That was at least how I felt a few years back when I couldn’t get rid of it. Luckily, as the years passed, I gained a lot of knowledge in this field.

Nitrates typically build up after an ammonia spike, when nitrifying bacteria convert the ammonia into nitrites, which later on, turn into nitrates. That usually happens in poorly maintained tanks with a lot of rotting organic matter.

As we move forward, I will share some excellent tips that will help you prevent nitrates from spiking in the future, including an excellent video on gravel vacuuming.

This dirty tank has a real potential for an ammonia and nitrate spike.

What Causes High Nitrates In Aquariums?

An article in Microbial Ecology (designed to study heterotrophic bacteria that remove nitrites from water) identified nitrates as a significant source of stress for fish, an element that weakens their immune system and exposes the creatures to infections.[1]

But you can’t fight high nitrate levels in an aquarium without first identifying the factors that attract the substance. 

The primary culprit is ammonia. Nitrate forms when bacteria turn ammonia into nitrites and nitrates.[2]

Technically speaking, this is a good thing. Ammonia is highly toxic. The cycling process introduces Nitrosomonas bacteria that convert ammonia into less harmful substances.

Cycling is slow. Initially, ammonia levels will rise. But as the population of Nitrosomonas bacteria grows, the ammonia concentration will fall. 

At the same time, your testing kit will reveal trace amounts of nitrites. Like ammonia, the nitrites will initially spike.

But as the bacteria perform their role, the nitrites will eventually fall as nitrate levels skyrocket. 

An aquarium is ready to accommodate fish once all the ammonia and nitrites have become nitrates. The more ammonia you have, the more nitrates you will detect. 

Common sources of ammonia, and thus, high nitrate levels, include rotting organic matter and poor maintenance routines.

It is worth noting that nitrates are less dangerous than ammonia. Cycling aims to protect fish by turning all the ammonia into nitrates. However, high nitrate levels are harmful to aquatic creatures.

Aquarists are expected to keep the concentration below 50ppm. Although, many professionals will tell you that 50ppm is still too high. 

You should aim for 25ppm or less. Better yet, keep nitrate levels below 10ppm, just to be on the safe side.[3]

Feed your fish the amount they can consume within two to three minutes to prevent unnecessary leftovers.

Why Do My Nitrates Keep Going Up?

High nitrate levels are a sign that something has gone wrong. If yours keep climbing despite your best efforts, keep an eye out for one or more of the following potential culprits:

1. Water Source

A paper exploring the influence of nitrites on fish (University of South Bohemia, University of Veterinary and Pharmaceutical Sciences, Czech Republic) found that ammonia, nitrite, and nitrate levels existed in lower concentrations in natural water bodies.

However, factories can increase nitrite concentration by polluting the water with metals and dyes.[4]

Where does your water come from? Aquarists respond to high ammonia, nitrite, and nitrate levels by performing a water change. But if the new water is tainted, your nitrate levels will climb instead of falling.

If your water comes from a tap, you’re in luck because you can ask your service provider for a detailed breakdown of the components in the water they send to your home.

If you favor a local water body, such as a lake, take a sample of the water to a lab. They will determine once and for all whether or not the water is the source of all your problems.

2. Fish Waste

When was the last time you cleaned the aquarium? Some newcomers respond to high nitrate levels by adding water conditioners. But water conditioners won’t help if the water has rotting waste and leftovers.

The decomposition process produces ammonia. If you cycled your tank, the Nitrosomonas bacteria will turn that ammonia into nitrates. 

Perform a physical inspection. Don’t rely on water changes to remove dead fish, decaying plants, and rotting food. 

3. Filter Media

If you have a strict maintenance regimen, but ammonia and nitrate levels are always high, the filter media is probably at fault. 

Waste and leftovers can clog the filter media, producing ammonia and nitrates after decomposing and undoing the results of your maintenance routine. 

Another thing to consider regarding the filter media is how well you’re cleaning your tank. Typically, this part contains essential bacteria that turn ammonia into nitrite and nitrate.

If you clean the filter media and accidentally remove that bacteria, ammonia will spike

Then, bacteria found in other parts of your aquarium will begin with the nitrogen cycle, although the byproducts will be higher than usual.

Nitrifying bacteria mainly thrive in the filter media, although they are also found in the substrate, air stone, and plants.

4. Overstocked Tank

Nitrate levels are more likely to skyrocket in overstocked tanks because the fish will overwhelm the water with waste and leftovers, especially if you combine overstocking with overfeeding.

As a rule of thumb, I suggest sticking to the one-inch-per-gallon rule. In other words, you’ll need one gallon for every inch of your fish’s length.

For example, if you have three fish that are two inches in length, they will need at least six gallons. Naturally, you’ll need more than that if you have plenty of vegetation and decorations.

How Do I Prevent Nitrates From Going Up?

While nitrates are not necessarily difficult to remove, you are better off preventing them from taking root in the first place. 

Yes, nitrates are less toxic than ammonia. But high nitrate levels can do lasting damage to your fish. Therefore, if you can keep the nitrate concentration below toxic levels, you should do so. 

Experienced aquarists will encourage you to take the following precautions:

1. Siphon The Substrate

Using a gravel vacuum is probably the most important thing when dealing with high levels of ammonia and nitrates.

Most of the rotting matter and fish waste are stuck inside the substrate. They will remain there and rot until you find a dedicated device capable of removing them.

Most aquarists use a gravel vacuum for that purpose. I personally use the Laifoo 5ft Aquarium Siphon Vacuum Cleaner (link to Amazon). But frankly, almost any gravel cleaner will do the trick.

If you’ve never used one before, here is an excellent Youtube video that will walk you through it:

2. Perform Regular Water Tests

First of all, you should test the water routinely. Keep a nitrate test kit on hand. Most testing kits come with instructions, although, they are not that difficult to use.

Then again, knowing how to test for nitrates is irrelevant if you can’t interpret the results, which is why the instructions are so important. 

If you have friends with experience in this field, don’t be afraid to consult them. However, you can also check my complete guide on testing aquarium water for nitrates.

Some people, including me, think 20ppm is acceptable. Others are convinced that anything over 10ppm is too much.

As for the frequency, I recommend testing the water every week. You can do this after each water change or cleaning.

The kit that I use is the API FRESHWATER MASTER TEST KIT (link to Amazon). This one is my favorite as it can last for eight hundred measures, making it extremely cost-effective.

It is key to test the aquarium water regularly to catch ammonia and nitrates spikes on time.

3. Add Live Aquarium Plants

If you’re new to aquariums, you might be surprised to learn that aquatic plants can lower nitrates and ammonia.

They use the substance as a food source, which is perfect because a healthy population of plants can keep nitrate levels under control. 

Don’t forget to install sufficient lighting. In the absence of light, plants are a threat to fish because they absorb oxygen. Therefore, you cannot solve your nitrate problem by crowding the tank with plants. 

Here are some great examples of aquarium plants that remove nitrates and are relatively easy to care for:[5]

  • Marimo Moss Ball (Aegagropila linnaei)
  • Amazon Frogbit (Limnobium laevigatum)
  • Hornwort (Ceratophyllum demersum)
  • Dwarf Hairgrass (Eleocharis parvula)
  • Water Wisteria (Hygrophila difformis)
Amazon Frogbit is an excellent plant that will help you lower nitrate levels.

4. Feed Your Fish Properly

Don’t overfeed your fish. Fish will eat whatever you give them. They don’t know how to say no to food. 

In fact, they may rush to the surface whenever they see you because they associate your presence with food.

Unfortunately, some aquarists take this as a sign of hunger and respond by adding more food to the water. 

Try to limit the fish to two or three meals each day. Give the creatures food they can finish within three minutes.

5. Clean Your Tank Properly

Keep the tank clean. Perform weekly water changes, and remove waste before it rots.[6] That includes the leftovers in the gravel. Make sure you vacuum the substrate.  

If you think the filter is dirty, clean it. Use conditioned water. Toxins like chlorine will kill the nitrifying bacteria in the filter media. Speaking of filter media, don’t remove the filter media.

It sounds like a good idea to replace dirty filter media, but doing so will leave your aquarium in an uncycled state.

Pro tip: Dealing with ammonia is key when you find high nitrate levels in your tank. Here is an article with all the information you need about ammonia, including the specific products I use to remove it.

If you found this article helpful, these may also interest you:


Nitrates are a by-product of ammonia, which usually spikes due to rotting organic matter and poor maintenance.

If your test kit found levels of 25 ppm or higher, it requires your attention. That is a strong indicator that your tank produces too much ammonia which is toxic to fish.

Start by vacuuming the substrate and performing routine water changes. Also, when cleaning, avoid washing the filter media, as it contains bacteria essential to the nitrogen cycle.

High levels of nitrates are less worrying than high levels of ammonia. Nevertheless, nitrates can also be toxic when they accumulate too rapidly.


  1. https://link.springer.com/article/10.1007/s00248-020-01498-6
  2. https://www.mdpi.com/2076-3921/11/3/545/htm
  3. https://www.thesprucepets.com/nitrates-in-the-aquarium-1381883
  4. https://www.agriculturejournals.cz/publicFiles/61325.pdf
  5. https://www.tankarium.com/best-aquarium-plants-to-reduce-nitrates/
  6. https://www.thesprucepets.com/nitrate-poisoning-in-aquarium-fish-1381288