How To Lower Nitrates In Reef Tank (Properly)

WolfmanSF, CC BY-SA 4.0, via Wikimedia Commons
WolfmanSF, CC BY-SA 4.0, via Wikimedia Commons

How To Lower Nitrates In Reef Tank (Properly)


Controlling the water parameters in your saltwater aquarium is crucial for it to thrive and one of the most important things to keep in check is the nitrate level. But what exactly are nitrates and why do they matter for the longevity of your reef?

Chemical Nitrate Reducers


Add to a hang-on filter or sump in a mesh bag to bind up excess nitrate.


Highly regarded nitrate and phosphate control liquid additive.

Nitrate Reducer

Alternative liquid nitrate reducer from a well-respected brand.

Nitrate Filter

Filter media pad that can be placed in the flow of your sump or hang-on filter.

What Even Are Nitrates?

Nitrates are essentially the end product of the nitrogen cycle and are the least toxic of the compounds. That does not mean that nitrates are safe though. They are still able to cause issues for your tank’s inhabitants if not properly kept in check. Though some corals can use nitrate for growth, they do not need high levels of it. Nitrate should be kept between .25 ppm and 5 ppm.

Some Issues Nitrates Cause for Your Aquarium

  • Coral can die or lose its color
  • High algae growth
  • Illness and death for aquarium inhabitants

What causes high nitrates in a reef aquarium?

Nitrates can commonly become a problem for reef tanks that are not quite prepared for the levels of nitrate present. For example, your aquarium might not have all the live rock and beneficial bacteria that would help to keep the nitrate levels in check. They can be caused by poor care practices such as overfeeding and infrequent water changes.

Avoiding High Nitrates Means Adhering to Best Care Practices

Avoid Overfeeding Your Fish

Food that is left uneaten and increased waste produced by fish will all begin to wreak havoc on the tank’s water quality. Not only can overfeeding cause a spike in nitrates in your aquarium water but it can also lead to increased ammonia levels. Try measuring the food for your fish so that they can eat it all in 1-2 minutes. This will help to cut down on the over-abundance of food present. If you don’t think you are feeding enough, you can always add more a little later. It is easier to add more food later than it is trying to remove excess food. And always try to stick to a schedule. I typically use a digital excel sheet to help remind me when and if I fed my aquarium that day. Whatever you need to do to be consistent in your practices, do it. By preventing overfeeding, you help make it easier to reduce nitrate levels naturally.

Frequent Water Changes

Perform your regular water changes to help remove excess waste, algae, and provide quality clean water for your fish. How much constitutes a good water change? Try to aim for between 15 – 30 percent for your weekly water changes. Ensure that the water you are adding back into your tank is safe and ready to mix with your aquarium’s water. That means no nitrates in the water (you know the compound we are trying to keep as low as possible). You can use filtered water and test the water before you actually begin to add it to the tank. You can buy nitrate test kits for your water changes if you use a nitrate monitoring system in your tank.

Add more Live Rock

One of the reasons your tank may have started with a nitrate imbalance is that it did not have the time to properly mature with bacteria before you began adding your fish. If you barely have any live rock, you should seriously consider adding more to your reef aquarium. Live rock comes with helpful bacteria that can be beneficial to a variety of things in your tank. One of those benefits is helping to cultivate the bacteria that work to reduce nitrates.

This should only be done if you did not take the time to properly add enough live rock in the first place. Throwing in an unnecessary live rock into your tank could cause more bad than good due to wasted space, blocked water flow, and algae buildup.

Protein Skimmer

A skimmer is always a great thing to consider if it is within your budget. They help to reduce the buildup of nitrates in the first place by cleaning up the waste and algae that it is derived from. They don’t directly reduce the nitrate level from the aquarium water instead they remove the compounds that lead to the production of nitrate. They are a great proactive tool to help keep your tank’s water quality perfect.

Get a good clean-up crew

A good clean-up crew will help to clean up a lot of the excess waste that might be missed by frequent water changes and your protein skimmer. Nas snails would make a great addition to your tank’s clean-up crew. They like to dig in the sand which causes it to be turned over and reveal waste that might have sunk into the sand bed allowing for it to be eaten or cleaned. Emerald crabs are also great for eating up algae and leftover food from your fish but you might need several for them to be successful. Careful not to overdo it and add too many cleaners to your tank.

Rinse off Frozen Food

Frozen food can be a great nutrient-dense way to feed your fish but sometimes these useful meals are packed with juices that are sometimes detrimental to your tank’s water quality. You should rinse these foods before feeding them to your fish. It will help to reduce the bad bacteria being introduced at the time and stabilize the nitrate level. The best way to do this is to use a fish net to hold the food and rinse them with filtered water. The same filtered water that you would be okay with using for your water changes, so guarantees that you will not be introducing harmful bacteria and water to your aquarium.

Remove Some Fish

You think you aren’t overfeeding your fish and you are performing weekly water changes, but your nitrate levels are staying at the same level. What this could mean is that you overstocked your saltwater aquarium. Also, you may not think you are overfeeding your tank, but chances are if the tank is overstocked, you very well could be overfeeding as well. You might need to remove a few of your tank’s inhabitants to stabilize the nitrate level.

How do you lower high nitrate levels in a reef tank?

You have followed the best practices laid out above for how to prevent your nitrate levels from spiking in the first place. But now you just have to reduce the nitrates that were increased from before. Here are several methods you can try but remember none of it matters unless you are going to take the time to remedy the issue that caused the increase in nitrates in the first place.

Water Change Method

A water change will help to reduce the nitrate levels by removing the water that is made up of too much nitrate and introducing quality water. A water change alone won’t fix the original problem if you have not already located it. If you need swift action, a water change will help but it is not the final answer, typically.

Consider using Chemical Additives

There are additives that you can use in the water to help reduce the nitrate levels. These chemical additives can work directly on the problem and keep even help with other chemical imbalances. The liquid chemicals that you add work best in tandem with a protein skimmer already working for your tank.   

An additive example would be no3 po4 x – Red Sea, which helps to prevent algae buildup by reducing its source. The chemical works to remove nitrates and phosphates from aquariums.

Biopellet Reactor Method

Next to water changes, the biopellet reactor can be one of the most effective practices you can introduce to your tank to control nitrates. The biopellets are actually made from bacteria and essentially as they breakdown help to lower nitrate levels as the water is pumped through the housing reactor.

I know you may be looking at the bag of pellets and thinking it must be a pain to replace all of that. However, you don’t need to replace the biopellets as they are consumed when they are entirely used up, so you only need to add more biopellets. What an easy life the biopellet reactor can be when it is properly utilized!

Biopellet Reactor Tips

Patience is a virtue for aquarists. Bio-pellets can be a huge benefit to your tank but if you are over-eager, they can lead to some easy to avoid problems. Avoid using all the bio-pellets in a single-use to avoid shocking your tank. You also need to have a protein skimmer working properly. Take a look at this article from Reef Tank Resource to decide what skimmer might be best for you, if you haven’t happened to invest in one quite yet.

Add only a small number of pellets to get started and have some manual tests available to test the water to see if it plateaus. If it begins to, you can safely begin to add a few more pellets until you begin to get the nitrate levels that you desire. This way you avoid over adding pellets and causing a shock to your tank.

Nitrate Reactor Method

A nitrate reactor, or denitrate reactor, is a device that helps to directly remove the nitrates from your water.

It works by running low flow water through its system and depriving oxygen in its chamber. What this does is help to cultivate bacteria that will consume the nitrates in the water system. You should be able to test the water in the chamber to see if the nitrates are being controlled. If the nitrates are lower in the reactor than they are in the main tank then your reactor is doing its jobs properly.

Many experts claim this is the most effective way of removing nitrates from your aquarium.

Refugium Method

A refugium is an additional chamber attached to your aquarium tank. The refugium might be used for various additional purposes like allowing organisms to thrive that might not survive in the main water system. However, for our immediate purposes, the refugium can help to regulate the nitrate levels of your tank. Consider adding a sand bed and live rock to the refugium to help act as an additional filter for your water supply.

There are many other benefits to using a refugium, so be sure to check them out so you can make the most of this useful piece of equipment.


As I have discussed, like many aspects of this hobby, the best offense is a good defense. You want to know what levels of nitrates you need for your corals, but you want to make sure it doesn’t get out of control too quickly. The best way to guarantee control is by frequent water changes and adhering to the best proactive practices.

If by chance, the levels become unmanageably high, you will need to choose what methods will work best for you to reduce them quickly to avoid damaging your reef community. What works best for you will depend on your own preferences so try out a few methods to what you prefer.

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