How to Reduce Phosphates in Saltwater Aquariums

How to Reduce Phosphates in Your Saltwater Aquarium

Every saltwater aquarium contains phosphates — otherwise, the coral and some other organisms that reside in it wouldn’t thrive and may even die. Phosphate is, after all, a natural part of seawater. However, there can be too much of a good thing. When phosphate levels grow too high, an abundance of algae develops inside the aquarium. It’s also stressful on your coral, can lead to stunted growth and calcification, and may even cause the coral to decay or die.

As a general rule, the phosphate level won’t impact your fish, but it can make for an unattractive aquarium. If your fish are doing well but your coral isn’t, it may be time to check the phosphate levels.

What Causes High Phosphates In Saltwater Aquariums?

There are many reasons why your saltwater aquarium may develop high levels of phosphate. There may be an abundance in the replacement water you use, especially if you use tap water. Adding too much fish food, especially food that goes uneaten, to your aquarium can also be the culprit. Other additives, like carbon and products used to stabilize your pH, can add to the phosphate level.

Many times, it’s not about what you add and more about what’s already in the aquarium. As living organisms and their byproducts, like plant material, fish feces, bacteria, and slime decompose, they release more phosphates out into the water.

How To Test For High Phosphates?

There are several devices available that allow you to test the phosphates in saltwater aquarium, or you can look for a general water testing kit. You’ll need a syringe or another item that allows you to collect and measure some water from the tank. You can then follow the instructions on whatever phosphate tester you choose, using the water you’ve collected. Many of them require you to add chemicals to the collected water, shake it, and let it sit for a few minutes before matching the color of the water to a color level on a chart.

What Are Normal Phosphate Levels In A Reef Tank?

Most of the time, we recommend keeping the phosphate levels in reef tank below 0.05 ppm. That’s similar to the levels in the ocean, which usually range from 0.05 ppm to 0.07 ppm. Be careful not to let your phosphate level go too low, especially below 0.02 ppm. This can actually starve your coral to death.

How To Lower Phosphates In A Saltwater Aquarium?

Now it’s time to learn how to reduce phosphates in a saltwater aquarium. While you’ll never fully get rid of the phosphate (and you don’t want to fully get rid of it), there are some quick and easy things you can to do to lower the level. Start by feeding your fish less, especially if they aren’t eating everything you give them. As food sinks to the bottom and decomposes, it raises the level of phosphates in a saltwater aquarium.

Changing the water can also help. You may want to check the water you replace it with before you add it to your aquarium, though. Some water is actually naturally high in phosphates. It’s also important to keep your aquarium clean, which means removing decomposing material frequently.

Another natural option is adding good algae — seaweed or macroalgae — to your aquarium. This option may not be for everyone for aesthetic reasons, but it can get the job done. You can also keep it in your sump filter to make it more visually appealing.

If you prefer chemical methods, there are some products on the market meant to reduce phosphate levels in a reef tank. These products absorb the phosphate in the water, but you must refresh them frequently for them to work. Many saltwater aquarium owners prefer products that contain granular ferric oxide. The ingredient forms a molecular bond with the phosphates and doesn’t release them back into the water as some other products do.


When learning how to reduce phosphates in a saltwater aquarium, you may try one or all of the methods mentioned above. In the end, it’s about which method you like the best, and which ones work best in your saltwater aquarium. After all, owning and maintaining an aquarium should be a fun hobby that doesn’t feel like hard work. Just be careful not to reduce the levels so that they are too low, and be sure to monitor your phosphate levels regularly. Higher levels can sneak up on you before you know it, and if you catch them in time, you can save your coral and keep your aquarium looking good.

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