How to reduce nitrates in your aquarium quickly and effectively

It’s one more thing to add to the seemingly endless aquarium checklist, but you need to stay on top of your nitrate levels. You’ll never get them to zero (though if you do, let us in on the secret), but keeping the aquarium chemicals in balance will greatly increase your fish’s life span. While not quite as dangerous as ammonia, out-of-control nitrates can kill fish. Proper testing, cleaning, and filtering will enable you to keep your nitrate levels low and your fish healthy.

Clown fish swimming through corals in an aquarium
Joshua J. Cotten /

What are nitrates?

Think back to 10th-grade biology, and you may recall the nitrogen cycle. In your tank, this is the process by which nitrogen moves through multiple forms into nitrate, the least harmful. It begins with fish waste or decaying plants that give off ammonia, which is extremely deadly and should be kept at zero ppm. Small bacterial colonies then turn this into nitrite (also harmful) and then nitrate (much better). Altogether, this cycle must occur, or your tank occupants would die from ammonia poisoning, but that does leave you nitrates to contend with. When you first set up your tank, the nitrogen cycle will occur to establish your bacteria and prep the water for your fish to thrive. Don’t panic if you notice cloudy water during this time — it’s all part of the process.

Why are they harmful?

Unlike with nitrites or ammonia, your fish will not die from a little nitrate in the aquarium. However, high levels can lead to problems, especially with their immune systems. In a high-nitrate situation, you may notice your fish slowing down and becoming lethargic or sickly. Extreme nitrates can even stunt fish growth or lead to skin problems such as sores. Young fish will suffer the most, and older fish will be much less likely to reproduce. Anything upward of 100 ppm should be taken as an emergency. Don’t forget, a chemical problem might indicate other issues, too, so you may see additional signs of stress in your fish not directly related to nitrates.

Brightly lit aquarium with fish and coral
PDPics /

How to monitor nitrates

As with other chemical levels in the aquarium, you’ll need to test frequently to keep your nitrates in check.

  • Before you even get your fish, check your water source, beginning with what comes out of your tap. It may exceed optimum nitrate levels already, in which case you should find new water.
  • When you first set up your tank and your nitrogen cycle hasn’t completed, you must run a test every two days. Once you’re stable, bump that down to every week or so, preferably when running a partial water change.
  • You can also look for signs that the nitrates have gotten out of control, like a particularly big algae bloom.

A freshwater aquarium may go along fine with a higher level than a saltwater aquarium would. We recommend under 10 ppm, though this can vary slightly based on your setup.

How to reduce nitrates in your aquarium

The best way to keep nitrates down is to keep your tank clean. This means promptly removing any old plants, deceased fish, or leftover food. Remember, fish waste and dead leaves will drastically increase many of your tank levels to unsafe numbers. Therefore, overfeeding and tank crowding cause big problems. Watch your fish at mealtimes and keep track of how much food goes to waste (also look into adding bottom feeders to your mini ecosystem). 

Plants in general aid in nitrate reduction, and when carefully maintained, will improve your tank overall.

Water changes are the most crucial piece of keeping a healthy aquarium. But don’t remove too much water, or you’ll have to start the whole process over again.

One more thing you can try: There are filters and special media to handle nitrate removal. If you find yourself with an excess week after week, check these out. Although they can be pricey, they’ll really help.

Like so many other parts of being an aquarist, nitrate levels require strict observation and a careful cleaning schedule. The most crucial time is when you start a new tank. Even aquarium pros sometimes lose fish when first creating a habitat. However, a little vigilance goes a long way, and hopefully, you’ll find your chemical composition keeping itself in check with minimal intervention from you after a few weeks.

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