How To Lower Phosphates In Reef Tanks (4 Best Methods)

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This is the complete guide to showing you how to lower phosphates in your reef tank.

If you want to grow and maintain healthy corals in your tank, it’s crucial to keep phosphate levels within limits.

Phosphate is one of the most important water parameters to test for and maintain in a reef tank.

Not only is it important to understand the effects of high phosphate levels, but you need to know how to adjust the level back into the proper range.

When lowering phosphate levels in a reef tank, you must do it slowly over time. A rapid change in phosphate levels can lead to a disaster with reef tanks.

There are 4 main methods to use when lowering the phosphate levels in a reef tank.

#1. Perform a water change and remove detritus

It is natural for a saltwater reef tank to have an increase in phosphates over time. One of the more common ways to correct any water parameter is to perform a water change.

As you remove water, you are taking phosphates out of the system and replacing it with fresh RODI water which will make a huge difference in the phosphate level.

You may want to consider doing an extra water change on top of your normal maintenance schedule and you will notice the phosphate levels begin to come down.

While you are doing the water change, also vacuum out any detritus, uneaten foods, and waste that is contributing to higher phosphate levels. 

#2. Filtration – protein skimmer

A protein skimmer is one of the most important pieces of equipment that you can have on a reef tank. Its purpose is to remove dissolved organic waste.

Having a good quality skimmer on your tank that is properly sized will be very effective in removing waste before it has a chance to break down and release phosphates into your system. 

#3. Macroalgae – algae scrubber

Another very effective way to lower phosphate levels is to use macroalgae in a refugium or add an algae turf scrubber into your system.

Refugiums are easy to set up and growing macroalgae inside will reduce and remove phosphate, nitrate, and other nutrients. Chaetomorpha is commonly used and is very effective. 

Using an algae scrubber will also reduce nutrient levels and works just like macroalgae in the refugium does.

Water will flow over an illuminated screen that contains turf algae which is then removed or harvested as it grows taking the nutrients with it. 

#4. Chemical products

There are many different products on the market that you can use to lower phosphate levels. Phosphate reducing filter media is a popular method to use and is also very effective.

It’s important to follow the directions of the media being used as using too much for long periods of time can have negative effects causing more harm than good. 

GFO or Granular Ferric Oxide is one of the products that can be used. It is normally used in a media reactor or filter bag and binds inorganic phosphate as water flows through it.

Compared to other products, GFO is best used for long term phosphate reduction. 

Other phosphate reducing products include:

  • Seachem Phosguard
  • Phos FX
  • Brightwell Phosphate E
  • RowaPhos
  • Red Sea NO3:PO4x

These products are available at Marine Depot here. 


Marine Depot Aquarium Supplies


What Is Phosphate?

Phosphate is a natural result of waste breakdown which includes uneaten fish foods, fish waste, and dead or decaying matter.

Over time, phosphates will build up in your tank and high levels will contribute to algae growth and starve the water of oxygen which causes an unhealthy environment for fish and corals.  

What should the phosphate level be for a reef tank?

The proper levels for phosphate in a reef tank should be:

  • Soft/LPS corals: 0.05 PPM
  • SPS corals: 0.03 PPM

reef tank water parameters


The phosphate level in your reef tank should be as consistent as possible and not fluctuating up and down. It’s important to not drop phosphate rapidly and never reduce it to 0.

Corals do need some phosphate in the water for them to grow and taking all of the phosphates out of the water is not advisable.

It is actually impossible to drop phosphate to 0 as most hobby grade test kits are not that accurate. It may show 0 but there is a small amount of phosphate that is unreadable by the test. 

Not all reef tanks are the same and some tanks will require more phosphate than others. So you may have 0.05 PPM in your reef tank with the corals doing great and in another tank, that level would need to be lowered for the corals to be healthy.

Find the right balance that works for your tank. 

Is high phosphate bad for reef tanks?

Yes, having a high level of phosphate can be a disaster on a reef tank. But the key word here is high. A normal level of phosphate that is within limits is not bad for a reef tank.

It is what you want to see. Science has proven that phosphate is not directly toxic to corals. It’s when phosphate concentrations are kept too high, it will have negative effects on corals.   


What causes high phosphate levels in reef tanks?

High phosphate levels in reef tanks come from 3 main sources.

1. Fish waste/decaying matter

As mentioned above, fish waste is a huge contributor to high phosphate levels. Any decaying matter and waste will release phosphate into the water.

The more fish you have in the tank, the more waste will be produced and more attention will be needed to keep the phosphate levels within limits.

2. Overfeeding/uneaten food

One of the biggest mistakes made by aquarium hobbyists is overfeeding their fish. Most people overfeed unintentionally and it directly affects water quality.

Overfeeding will also result in uneaten fish food which will break down and release phosphate into the water. 

3. Poor water quality/tap water

When keeping a reef tank it’s important to only use RODI water for both water changes and freshwater top-offs.

Tap water should not be used as it contains harmful impurities including phosphates which are harmful to fish and corals.

Poor quality tap water is a main contributor to high phosphate levels in a saltwater reef tank.


How to test for phosphate

When testing for phosphate levels in your reef tank, it’s important to use a good quality test kit that will give you accurate results.

Some test kits are not that accurate and trying to read color charts can be difficult and confusing. Phosphate requires a test kit that is more precise and easy to read.

I recommend using the Hanna testers as they give you a digital readout with no color charts. 

2 separate Hanna testers are available for testing Phosphate levels.

Hanna tester comparison – Marine Depot


  • The standard low range tester is designed for fish only and soft coral tanks and has a range of 0 – 2.5 PPM with an accuracy of plus or minus 0.04. 
  • The Ultra-low range tester is designed for mixed reef and SPS tanks where more precise readings are needed. It has a range of 0 – 0.9 PPM with an accuracy of plus or minus 0.02. 

Recommended Phosphate checker

hanna phosphate tester


Using any of the methods mentioned here will help lower phosphate levels in your reef tank.

By using a routine maintenance schedule, keeping your fish bioload low, not overfeeding your fish, and using high-quality water, will ensure you are on the right track to having low phosphate levels.

There are also some great products like GFO which can be very helpful in the long-term goal of keeping phosphates low in your reef tank.