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What Are The Signs Of High Nitrates In Aquariums?

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As a fish owner, I knew that toxins like nitrate can be lethal to aquarium fish, especially in high concentrations. Unfortunately, for many years, I didn’t know what signs are associated with this condition.

These signs typically indicate that nitrate has spiked in your tank:

  1. Algae will increase in your tank for no apparent reason.
  2. Your fish will become lethargic and be less interested in food.
  3. The mortality rate in your tank will rise.
  4. Young fish will stop growing properly.
  5. A testing kit will be positive for nitrates.

As we move forward, I will elaborate on the different signs listed above. Then, I will share a few tips on what you should do if nitrate has spiked in your tank and include the products I use to fight that issue.

What Are The Signs Of High Nitrates In Aquariums?

You cannot avoid nitrates because ammonia is a common aspect of aquariums, and nitrates appear when bacteria convert ammonia into nitrites and then nitrates. 

If you have an established tank populated by living creatures, you can safely assume that the water has nitrates. 

Your biggest concern isn’t the presence of nitrates but rather the concentration. Nitrates are technically less toxic than ammonia. However, high concentrations are still dangerous. 

Because the substance is colorless and odorless, you can only identify high nitrate levels by looking for the following signs:

1. Your Testing Kit Will Be Positive For Nitrates

A testing kit is more accurate than any of the signs you usually use to identify high nitrate levels.

Some people prefer comprehensive testing kits that investigate various elements, including water hardness, ammonia, and nitrites. However, testing kits that only search for nitrates are much cheaper. 

Paper strips are the least accurate of the bunch. You use these devices by dipping strips of paper in a sample of the aquarium’s water. The strip will change color. 

You determine the nitrate concentration by comparing the color of the strip to the chart that came with the testing kit. 

Liquid kits are somewhat superior. They include a testing fluid. It only takes a few drops of this testing fluid to change a sample of the aquarium water to a color that reveals the nitrate concentration.

As you may have guessed, this kit is also accompanied by a chart that reveals the meaning of each color.

Liquid testing kits are typically more accurate and last longer than simple strips.

2. There Will Be More Algae In Your Tank

Many people associate nitrates with their impact on the health of fish. They don’t realize that nitrates and algae have a close relationship.

Algae is dangerous to fish because it can create an imbalance in your tank’s ecosystem by consuming vital nutrients and resources.

This is a problem because high nitrate levels promote algae growth. 

A 2021 paper in the National Library of Medicine investigating the efficacy of green microalgae as a nitrate removal tool (Vaishali Rani and Gergely Maroti) found that microalgae thrived in the presence of various nitrate concentrations.[1]

An earlier study from 2015 (Ferdowsi University of Mashhad, California State Polytechnic University) identified algae as a suitable tool for removing nitrates because algae assimilate nitrates.[2]

Therefore, you wouldn’t be wrong in interpreting an unexpected algae bloom as a sign of high nitrate levels. 

It is worth noting that plants absorb nitrates as well, and they will thrive in the presence of the substance.[3]

However, if the nitrate concentration is too high, the plants will suffer because the algae will grow at an unprecedented rate.

Algae may flourish in tanks with no apparent lighting changes due to ammonia, nitrite, and nitrate.

3. Your Fish Will Manifest Unusual Signs

Human beings interact with nitrates all the time. Drinking water in the US has nitrate levels as high as 40ppm.[4] This is why water conditioning is so important

As a side note, nitrates should be kept below 20 ppm in aquariums. Levels of 40 to 50 ppm and higher can be pretty toxic to fish.

Many aquarists use water changes to combat nitrates. But some of them will inadvertently make things worse with the water change by adding tap water with significant nitrate concentrations. 

Humans are not immune to the negative consequences of nitrates. 

A 2005 paper investigating the impact of nitrates in drinking water on human health identified various epidemiological studies that blamed elevated risks of colon cancer on significant nitrate concentrations in drinking water.[5]

A CBS news article highlighted the work of a nonprofit organization (Environmental Working Group) that connected nitrates in drinking water to 12,000 cases of cancer.[6]

Human beings are larger and more robust than fish. Therefore, if nitrates can adversely affect human health, what do you expect the substance to do to fish? 

Fish in a tank with high nitrate levels will manifest various side effects, including:

  • The creatures will become lethargic. Whether they lie still at the bottom or hover at the top, you will quickly notice that your fish have little or no energy. 

Rather than darting wildly all over the place, they will find a safe location and settle because they are too sick or stressed to move.

  • Their colors will change. Many fish change color in response to stress. 

High nitrate levels are unlikely to produce the same burns you see in tanks with ammonia. However, don’t be surprised if the fish lose their colors. 

  • Some fish will become lethargic in response to high nitrate levels. Others will do the opposite. They will swim erratically. They may even hover upside down. 

Although, you are more likely to observe cases of glass surfing. Nitrates can damage the swim bladder, a development bound to exacerbate erratic swimming.

  • Their breathing will become labored, the gills moving rapidly in response to the shortness of breath.
  • Fish in a tank with high nitrate levels are unlikely to eat. 

As they succumb to listlessness, their interest in food will gradually fade until they stop eating altogether. Fish that stop eating will die sooner rather than later unless you remedy the problem.

  • The fish may develop crooked spines. Or, at the very least, their bodies may appear to curl from head to toe.
Some fish will respond to nitrate poisoning by staying motionlessly at the bottom of the tank.

4. The Mortality Rate Will Suddenly Rise

Your tank’s inhabitants will die if the high nitrate levels persist. Besides lowering their immunity, nitrates will induce stress in the various animals that call your aquarium home. 

It can take the highest nitrate levels 24 hours to kill your fish. However, most people have several days or weeks to act before their aquatic pets die.

This isn’t surprising. After all, nitrates typically infiltrate the gills before entering the bloodstream and wreaking havoc.

If your tank’s inhabitants are dropping dead left and right for no apparent reason, take that as an encouragement to test the water. A testing kit will tell you once and for all whether the nitrates are to blame.

5. Young Fish Will Stop Growing

High nitrate levels will stunt the growth of young aquatic animals.[7] Additionally, the substance will saddle young fish with health complications they will carry for the rest of their lives. 

Don’t be surprised if swimming becomes a difficult task for juvenile fish. 

If this doesn’t concern you because you don’t mind euthanizing your young fish and starting over, you should also realize that high nitrate concentrations can inhibit reproduction.

What Should I Do With High Nitrate Levels?

First, it depends on how high the nitrates are. As mentioned earlier, the most accurate way to measure aquarium toxins will be using a testing kit.

For that purpose, I use the API FRESHWATER MASTER TEST KIT (link to Amazon). It’s a highly accurate liquid test kit that lasts for about eight hundred measures.

If the nitrate is below 20 ppm, you shouldn’t necessarily make drastic changes in your tank. It’s a perfectly acceptable level.

However, levels of 40 to 50 ppm or higher require your intervention. In that case, the first step would be to conduct a significant water change, of about 40 to 50 percent.

Before adding tap water to your tank, make sure to condition it properly. You can easily do that with a water conditioner such as the Seachem Prime Conditioner (link to Amazon).

All you have to do is to add 1 m/L of this product for every 10 gallons of water. This conditioner will remove chlorine and chloramine, and detoxify ammonia, nitrite, and other heavy metals.

The Seachem Prime conditioner won’t directly remove nitrate. However, it will eliminate ammonia, its precursor. Doing so will prevent nitrates from forming in the first place.

Vacuuming the substrate is another crucial step when fighting toxin spikes, including nitrate.

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.

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Nitrates can become toxic when reaching concentrations of 40 to 50 ppm. Unfortunately, some areas feature that precise concentration in tap water.

For that reason, it is essential to know the signs associated with nitrate poisoning in aquarium fish. First, you will notice some behavioral changes.

Some fish will become lethargic and spend most of their time at the bottom of the tank. Others will swim erratically. Young fish may even stop growing altogether.

To prevent the situation from deteriorating, I suggest checking the aquarium water once a week. Also, it is crucial to use a conditioner when conducting a water change.