Reef Tank Phosphate Levels (Coral vs Algae Growth)

Image Credit: City.and.Color via Flickr
Image Credit: City.and.Color via Flickr

Reef Tank Phosphate Levels (Coral vs Algae Growth)


The level of phosphates in your aquarium is another one of those phrases that you have probably heard a lot about but are not quite sure exactly what it means for the overall aquarium health. Phosphates are simply a compound of phosphorus and are a crucial part of any reef environment. However, the problem can be when there is too much or too little phosphate in your tank’s system.

At Reef Tank Resource we are devoted to guiding aquarists both new and old onto a path where they can properly care for their aquarium’s population. Phosphates are just one piece of the large puzzle that an aquarist must begin to piece together to keep their fish and corals happy and healthy.

Reef Tank Phosphate Levels

Reef aquariums should keep phosphate levels very low but not zero. Corals use small amounts of phosphate for growth.

For reefs that include LPS (large stony polyp) corals, the phosphate level should be below 0.05ppm.

For reefs that include SPS (small stony polyp) corals, the phosphate level should be below 0.03ppm.

To better understand the benefits and drawbacks of phosphates and how much you really need in your aquarium, we will go over the differences in levels for different setups. We will also cover how phosphate interacts and leads to the increase of algae in your tank’s system. To ensure proper water quality, it is crucial to understand the steps you need to take to promote healthy growth.

What should my phosphate level be?

It isn’t as simple as knowing where your phosphate level should be. Phosphates are more of a range to help guide where you should set them for your tank. Depending on the type of reef tank that you have, you will want to avoid exceeding a certain maximum phosphate level.

For fish-only aquariums, you have a little bit of wiggle room where your phosphate level can be as high as .5 ppm. LPS corals set-ups shouldn’t exceed 0.05 ppm and SPS coral tanks shouldn’t exceed 0.03 ppm. These levels are lower because the corals will be actively using the phosphates in the tank for their growth, which is also why it is important to ensure there are some phosphates in the tank so they can continue that healthy growth.

Do corals need phosphate?

Yes, corals do need some phosphate. Phosphates help in healthy coral growth. If you are trying to create a completely phosphate-free environment, you are placing your corals in danger. That is why it is important to become familiar with how to create a stable and low supply of phosphates in your reef tank.

Phosphorous is important for coral in order to grow tissue. However, consider what kind of tank you are curating. If your aquarium has fish present then you should have many issues but if you are only caring for coral, then you might need to find a way to supplement phosphate into your aquarium.

You can help add to the phosphate level in your tank by using a commercial additive. Just make sure that you are not overdoing it and creating a heavily rich phosphate water supply. You might have bigger problems than lack of coral growth then.

Do phosphates cause algae growth?

If there is one thing you take away from this article, it’s that, yes, the level of phosphate in your aquarium will dictate algae growth. The increased algae in your tank not only looks bad, but it can also lead to reduced oxygen in the water. The algae can also create their own toxins depending on the type of algae that is growing in your aquarium.

It should go without saying but if you are dealing with an algae bloom, you should target its food source in conjunction with targeting the algae. If you only try to remove the algae from your aquarium through manual to chemical means, there is a likelihood that the remaining phosphate will only allow for the establishment of another algae bloom.

Will high phosphates kill fish?

It isn’t the phosphate that is the problem for the fish but it is what the high phosphate level causes in your tank that is the problem. Phosphates help promote algae blooms and algae blooms. The increased algae population will take oxygen from the water and can result in the death of your fish. The level of phosphate might not directly be the cause of death for your fish but it certainly is a factor.

What causes phosphates in a saltwater tank

Phosphate is released into your reef tank through fish waste, decaying food, dead algae cells, and other organic material can all contribute to the level of phosphate in your reef tank. This is why it is important to both not overfeed and properly clean your aquarium regularly. These practices help to reduce the overall buildup of phosphate over time.

An increase in phosphate can even be caused by the type of water that you introduce into your reef aquarium. Tap water from many cities and boroughs can actually directly introduce phosphates in your tank, which is why it is important to use RO/Di treated water to prevent any unnecessary additives to the system.

How to test for phosphates in aquarium water

Testing for phosphates is an important part of maintaining a healthy aquarium system. There are different test kits available depending on the type of aquarium that you are curating. You will want an accurate test that can measure closely enough to get a reading that is helpful for coral growth. Test kits that use the orthophosphate analysis method will ensure that you are able to get a clear understanding of your tank’s phosphate range.

Just make sure that you are getting a test kit specifically for your type of aquarium. If you are running an SPS tank, you will want a much more accurate test kit; compared to if you were only curating a fish-only reef tank.

How to lower phosphate in saltwater tanks

Of course, there will be times when phosphate levels begin to become too high. In order to avoid algae blooms and other problems for your fish inhabitants, it is good to know how to lower those levels effectively.

  • Protein skimmer – If you have the appropriate protein skimmer for your reef tank, you will significantly reduce the waste that ultimately builds up and creates the high phosphate level in the first place. Just make sure that you are emptying the collection cup regularly to ensure the skimmer continues to work effectively.
  • Regular cleaning and water change – Water changes will help to reduce aquarium phosphates by directly removing the water from the system. Make sure you are replacing the water with RO/DI clean water that has been properly mixed with salt. Also, the most effective cleanings have you vacuuming the substrate to remove any excess waste or fish food that might be resting there. Depending on your clean-up crew this might not be as necessary all the time but there will still be times where you should manually clean the bed of the aquarium.
  • Macro algae – Another great way to help reduce the level of phosphates in your aquarium is to provide more organisms that will help to control their spread. If you can, creating a macro algae farm in a refugium in a place like your sump will help to keep the phosphate levels in check.
  • Chemicals – There are several products available that will help to reduce the level so phosphates in your reef tank. Products like granular ferric oxide can help to drastically decrease the phosphate quickly and some can even have dual purposes so make sure that you are reading each product carefully to ensure that you are using the best one for your current situation. Be careful at the amount of phosphate removal you are actually doing. Some products can aggressively remove phosphates which can result in shocking the inhabitants of your tank.

How to raise phosphate in reef tanks

The most effective way without dirtying up your tank is to use a commercial additive that will directly contribute to the increase of phosphates in your reef aquarium. Whatever you do, don’t just try to overfeed your fish or add tap water to try and increase the phosphate level in your aquarium.

The long-term effects of those practices will only hurt the environment of your aquarium and won’t be worth the trouble. Using a commercial additive is a much more controllable method and allows you to know exactly what you will be affecting in your aquarium.

What is the best phosphate remover?

There are a lot of good options for phosphate removal on the market but here are some of the standouts.

  • Two Little Fishies Phosban – Will remove silicate and phosphates from the water without releasing other substances into your tank’s system.
  • Kent Marine Phosphate Sponge – Will help to remove phosphates down to .05 ppm if placed directly in water flow.
  • Seacham Phosguard – Will help to remove phosphates and silicate from the water supply. Can remove as much as 20 to 60 mg/l of phosphates in a 100-gallon tank.

These are only a few options that you can consider to help reduce the level of phosphates in your tank.

Will a water change lower phosphates

Performing a water change is one of the most effective ways to reduce toxic substances from your reef tank’s system. Removing the water that has high levels of phosphate will reduce the levels but there are a few things to keep in mind when you are carrying out this practice.

The first is that the water that you choose to replace the tank water with si incredibly important. regular tap water mixed with salt will not be sufficient in providing a healthy environment for your tank. One of the most common problems with the water is that it has phosphates in it, so using water from the tap kind of defeats the purpose of the water change if phosphate removal was the only goal. You should use RO/DI treated water when performing a water change. You can get this water at pet stores or you can make it yourself if you have a RO/DI unit.

The second thing to keep in mind is that all of this that if you are not dealing with the source of the phosphate spike, everything that you are doing is for naught. Why is there a phosphate spike? Is there excess fish food in the tank from overfeeding? When was the last time you cleaned the tank? Are there dead fish or waste sitting in dead zones? These are all things you should try to ensure are taking care of in conjunction with your water changes so they can be as effective as possible in improving the quality of your tank’s water.

Do protein skimmers remove phosphates?

When protein skimmers are set up appropriately, they can be extremely effective in removing waste and other harmful particles from an aquarium’s water supply. These particles are what contribute to the phosphates in the water in the tank.

The only caveat is that while the protein skimmer will help to remove phosphates and the waste where it comes from, it can also remove beneficial trace elements. It might be prudent to keep additives on hand to supplement the necessary trace elements that your tank needs to thrive like magnesium and calcium on hand.

Why can phosphate be bad for reef aquariums?

As we have covered. having phosphate present in reef aquariums is necessary for the tank to be healthy; however, it is also possible to have too much in the aquarium. The level of phosphate in your tank will help to promote not just the growth of corals but also algae. Green hair algae for instance can bloom when the phosphate levels of a tank are out of control which can cause quite a few headaches. Brown algae too can see an increase and grow directly on your coral.

The last thing you want is for these algae blooms to take necessary nutrients from the other organisms in your tank. Plus, hair algae and brown algae don’t make an aquarium look aesthetically pleasing.

Will carbon remove phosphates?

Carbon can help to reduce nitrate and phosphate levels in your aquarium, but correct dosing is crucial. Carbon can be introduced into a tank by doing your aquarium with alcohol. Most aquarists use vodka as their dosing alcohol of choice. As you might imagine, dosing vodka uses very little of the liqueur. The dosing range is between 0.1 and .9 ml of vodka per 25 gallons of water.

However, there is another method of introducing carbon into your aquarium without dosing vodka. Some reactors force water through a carbon filter before it goes through the rest of the plumbing in your aquarium. The difference is that this will work all the time but the problem can be that since it is working all the time, phosphate levels might be too low. It is almost too effective in reducing nitrate and phosphate levels in your tank but depending on your circumstances that might be a good thing.

How long does GFO take to reduce phosphates?

Granular Ferric Oxide (GFO) is a great way to help reduce the phosphate in your reef aquarium. If properly used, the GFO should reduce your phosphate problem within 4-8 weeks of starting.

To best use GFO in your own tank, follow these steps.

  1. Measure out about 1 tablespoon of GFO per 4 gallons of water.
  2. If you are using a filter bag, place the measured contents into the filter bag and completely close. Place the bag in an area of your aquarium with high flow.
  3. The level of phosphates should begin to drop but you might see an increase again within 8 weeks. If that occurs, just replace the media and do it again. You will be able to maintain a lower phosphate level this way but you should begin to see if there is a reason why your phosphates keep rising. Make sure there isn’t excess food or waste in the tank anywhere.

Final thoughts

Learning to maintain the optimal level of phosphate in your aquarium is crucial to ensuring safe conditions within your tank while also promoting the growth of coral in the long term. There is a delicate balance between too much phosphate and not enough, so make sure that you are doing what you can to maintain a stable environment.

The biggest thing that you can do is to implement regular care practices that ensure that you are removing excess food and waste. Performing water changes, investing in a protein skimmer, testing the quality of the water, and having emergency phosphate removal additives on hand are all great practices to implement into your aquarium care.

If you found this article helpful, I highly recommend checking out our article that covers the 8 main water parameters that you should monitor in a reef tank.

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