Is Nitrate Good or Bad for Your Aquarium?

If you search online for information about nitrate in fish tanks, many articles immediately pop up to warn you about the dangers of high nitrate and the best methods for lowering it. But how much nitrate is considered dangerous? And if nitrate is so toxic, why do many aquarium plant fertilizers increase nitrate levels when they are made to be safe for fish, shrimp, and snails? Let’s talk about one of the main points of confusion in the aquarium hobby — nitrate.

What is Nitrate?

When fish and other animals eat food and poop in the aquarium, their waste produces toxic nitrogen compounds like ammonia. Beneficial bacteria in the fish tank naturally grows and consumes the ammonia, purifying the water in the process and making it safe for fish to live in. However, one of the end products generated by the beneficial bacteria is nitrate. Nitrate is significantly less toxic than ammonia, but in large amounts, it can also start to negatively impact animals. For more information, read The Easy Guide to the Nitrogen Cycle for Aquariums.

How to Measure Nitrate

Nitrate cannot be detected by the naked eye since it is both colorless and odorless, so fishkeepers usually measure it using either water test strips or kits that chemically react to the nitrate in the water. For example, Aquarium Co-Op Multi-Test Strips quickly measure nitrate and five other parameters in just a minute. Simply dip a test strip in the aquarium water, such that all the test pads are submerged, and gently swirl it underwater for 3 seconds. Then remove the test strip out of the fish tank without shaking off the excess water and keep it horizonal for 60 seconds. As soon as the time is up, immediately compare the results with the included color chart to read the nitrate amount.

Use multi-test strips for measuring nitrate and other water parameters.

What are Safe Levels of Nitrate in Aquariums?

While some nitrogen waste compounds like ammonia and nitrite are toxic to animals at even trace amounts, nitrate is considerably less toxic. However, little research has been done to determine how toxic nitrate is to all of the different animals we can keep in our aquariums. As a frame of reference, a research paper titled Nitrate toxicity to aquatic animals reports that nitrate concentrations were raised up to approximately 800 ppm before they became lethal to guppy fry. We personally recommend keeping less than 80–100 ppm nitrate in your fish tanks.

Many people see this upper limit for nitrate and assume that, for the health of their aquarium animals, it would be best to lower nitrate as much as possible. While fish, snails, and shrimp are not affected by the lack of nitrate, live aquarium plants absolutely need it to grow well. When nitrate drops to 0–20 ppm, plant leaves can turn yellow or translucent (especially starting at the leaf tips) and eventually melt away because the plant is forced to consume nutrients from its old leaves at the bottom in order to make new leaves at the top. Therefore, we aim to keep approximately 50 ppm nitrate in planted tanks.

Signs of nitrogen deficiency

How to Lower Nitrate in High Bioload Tanks

If you have a fish tank that is heavily stocked with animals or does not have a lot of aquarium plants, the nitrate level produced by fish waste can naturally climb to 80–100 ppm and above. The fastest, short-term way to lower nitrates is to physically remove it from the aquarium by doing a partial water change. Take out 30–50% of the old, nitrate-laden water using an aquarium siphon and refill the tank with fresh, clean water. Generally speaking, we want to avoid shocking the fish by doing huge water changes, so if your nitrate level is far greater than 100 ppm, you may need to do multiple water changes over the course of several days to safely lower the nitrate. For a step-by-step guide on how to do this, see our flow chart for water changes.

Since most people don’t enjoy doing frequent water changes, let's look at some approaches for keeping nitrate levels lower in the first place. High nitrate is often seen in aquariums with high bioload — meaning that lots of fish poop, dead leaves, leftover food, and other rotting organics are in the water. Hence, the easiest methods to reduce nitrate in the long term include decreasing the number of fish and/or amount of food that goes into the tank. If you’re not keen on reducing your fish population, then try upgrading them to a bigger aquarium or adding large quantities of live plants. We love aquatic plants because they naturally consume nitrate as food, allowing them to grow more leaves and roots. In general, fast-growing plants like water sprite and Pogostemon stellatus are capable of eliminating nitrate at a quicker rate than slow-growing plants like anubias and java fern.

Is Fish Poop a Good Enough Fertilizer for Aquarium Plants?

Besides light and water, plants require an exact mix of nutrients to give them the fundamental building blocks needed to survive and thrive. Macronutrients are nutrients that plants consume in significant quantities (such as nitrogen, phosphorous, and potassium), whereas micronutrients are nutrients that plants need in trace amounts (such as iron, boron, and manganese). Traditionally, it was thought that fish poop and uneaten fish food were sufficient sources of nutrients for plant growth, but in reality, they do not contain all these necessary nutrients in the right ratios or amounts. When beginners try to keep plants without any fertilizer, the plants often develop serious nutrient deficiencies within a few months. To address this common problem, we developed Easy Green as an easy, all-in-one fertilizer to help keep plants healthy and well-fed.

As you can see in the list of nutrients above, the purpose of Easy Green is to raise nitrate (or nitrogen) and other nutrients so that plants have enough to consume. In fact, the percentages of nitrate, phosphate, and potassium are higher than the rest because they are macronutrients that your plants need in greater amounts. As a result, adding Easy Green will increase nitrate when measured by a water test strip or kit. In fact, the goal is to dose enough Easy Green until the nitrate level reaches 50 ppm.

How to Keep the Right Amount of Nitrate for Aquatic Plants

How do we reach the ideal concentration of nitrate without having too much or too little? If your planted aquarium consistently has too much nitrate, you may be tempted to stop using Easy Green since it will further increase nitrate levels. However, withholding fertilizer may end up depriving the plants of other essential nutrients besides nitrate. To prevent this from happening, use the following instructions:

1. If nitrate is 50 ppm or above, do a 50% water change (or multiple 50% water changes every four days) until nitrate reaches 25 ppm at most.
2. Dose 1 pump of Easy Green per 10 gallons of water. Wait a few hours and test the water again.
3. The goal is to reach 50 ppm nitrate. If nitrate is still too low, repeat Step 2 to keep dosing fertilizer until you reach 50 ppm.
4. Wait 3–4 days and test the water again. If nitrate is already at 75–100 ppm, you will have to do another 50% water change. Consider removing some fish or adding more plants (especially fast-growing ones) to decrease the rate at which nitrate is accumulating.

Quick dosing with Easy Green all-in-one fertilizer

On the other hand, if your planted tank always has too little nitrate, you should regularly dose fertilizer to avoid starving your plants. As a starting point, we recommend dosing 1 pump of Easy Green per 10 gallons of water with the following frequency:

• Dose once a week for low light aquariums.
• Dose twice a week for medium light aquariums.

If you find that your plant leaves are still developing holes and melting away, a customized dosing method may be needed, based on the nitrate level of the water.

1. If nitrate ranges from 0–25 ppm, then add a full dose of Easy Green (according to the instructions above). Wait a few hours and test the water again. 
2. If nitrate is still below 50 ppm, repeat Step 1 and keep dosing fertilizer until you reach that level.
3. Wait 3–4 days and test the water. Dose Easy Green again if needed to reach the goal of 50 ppm nitrate.

Record down the dates you fertilized the tank and the amounts of Easy Green used, and soon you should be able to figure out your custom dosing schedule. If you are unable to dose enough fertilizer to reach the nitrate goal, try decreasing the amount of lighting and/or CO₂ injection and repeat the previous steps. Also, be aware that as plants and fish grow bigger or are removed from the aquarium, this changes the amount of nitrate that is needed, so keep an eye on the growth of the plants and test your water to adjust the schedule as needed.

Bottom line: do not be alarmed if you see nitrate readings higher than 0 ppm. Nitrate is good and even necessary for plants. That is why we created Easy Green as a beginner-friendly fertilizer so you don't have to measure out a lot of supplements. Just add 1 pump per 10 gallons and watch your plants grow.

For more information, see the following articles:

• Which Planted Tank Fertilizer Is Right for You?
• Nutrient Deficiencies: Why Your Aquarium Plants Are Dying