All You Need to Know About: Phosphate

All You Need to Know About: Phosphate

Both corals and invertebrates need small amounts of phosphate; however, high levels inhibit coral growth and coloration and lead to nuisance algae. Phosphates in a reef tank can quickly get out of control, and learning how to manage them is key to a successful reef tank.

What Is Phosphate?

Phosphate is a naturally occurring compound consisting of a central phosphorus atom surrounded by four oxygen atoms. When organic substances, like fish waste and fish food, break down, they release phosphate into the water column.

Why is Phosphate Important? 

Proper levels of phosphate are essential for coral coloration and tissue growth in both corals and invertebrates. If levels become too low or undetectable, it can lead to coral bleaching and eventually death. Low levels can also lead to the proliferation of dinoflagellates. When these levels along with nitrate are too low, dinoflagellates can quickly take over as there is nothing to outcompete for resources with them. On the flip side, high levels encourage brown algae growth causing the coral to turn brown. High levels can also stunt coral growth by restricting the uptake of calcium carbonate.

What is the Optimal Range?

Reef Aquariums: 0.02 – 0.05 ppm

Tools for Measuring Phosphate

Here are the most common instruments for measuring phosphate in a reef tank, priced high to low. Please keep in mind that measuring these levels in an aquarium with an algae issue is challenging as the algae immediately consume new phosphate. It is generally safe to assume that if you have nuisance algae that these levels are too high.


A photometer measures the intensity of light. As the concentration of phosphate increases within a water sample, the deeper the color becomes. The photometer measures the color of the water and translates it into a reading of the level. A photometer is easy to use and displays results on a digital readout.

Hand-held Tester

A colorimeter or hand-held tester also measures a water sample’s color and translates it into a value. Compared to a photometer, it is relatively inexpensive.

Test Kit

Test kits are generally the most inexpensive and most commonly used option. A sample of water is color-matched on a chart to determine levels.

How to Maintain Phosphate Levels

Here are the most common ways to lower and maintain phosphate levels:

1. Regular Water Changes:

Regular water changes remove a percentage of water and replace it with phosphate-free water. Increase the frequency of water changes until levels return to normal. Test RO-DI water regularly to ensure it is phosphate-free and change all RO-DI filters when needed.

2. Do Not Overfeed:

Fish that eat more will produce more waste which turns into phosphates as it decomposes.

3. Use a High-quality Protein Skimmer:

Using an efficient protein skimmer will remove fish and other tank waste before it can break down into phosphates.

4. Use GFO:

Granular Ferric Oxide (GFO) media absorbs phosphates, chemically bonding them to the ferric oxide. For optimal results, use a media reactor and tumble the GFO slowly. Replace the GFO when exhausted.

5. Add a Refugium:

Add macroalgae to an adequately sized refugium. Macro-algae will consume phosphate, naturally keeping levels in check.

6. Start a Carbon Dosing Regime:

An organic carbon source, like liquid carbon or bio-pellets, provides food for good bacteria in the system. As these bacteria multiply and grow, they consume nitrates and phosphates from the water. An efficient protein skimmer removes the excess bacteria from the water column.

What is the Relationship Between Phosphate and Nitrate?

Phosphate and nitrate levels are interconnected. If phosphate levels are too low, the good bacteria in the aquarium cannot efficiently consume nitrate. The same is true for nitrate. If nitrate levels are too low, the good bacteria in the system cannot efficiently consume phosphate. The ratio of these levels to carbon is known as the Redfield Ratio.

Learn More About the Redfield Ratio:

There you have it! Properly managing phosphate levels is one of the most common problems faced by reef hobbyists. Maintaining the proper levels will prevent algae outbreaks and help keep corals colorful. Regularly testing your reef tank and RO water is the first step to keeping your levels in check. We highly recommend checking out our water care products here as well if you haven’t already. Happy Reefing!

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