How to Lower Phosphates in a Reef Tank: Mastering Nutrient Control

Phosphate has always been a hot topic with reef keepers. It’s in our tap water. It’s in fish and invert food.  Phosphate gets the blame for algae problems. Some say it even hurts corals.

When starting out, you’ll find a lot confusing information on the web, but don’t worry. We’ll break it down and tell you everything you need to know about phosphate and how to control it in your aquarium.

What Is Phosphate?

Phosphorous is an essential element needed by all aquatic life.  It’s a “building block” for cellular energy and it’s a component of essential hormones and enzymes.

Fish and invertebrates consume foods like plankton, algae and meaty foods that are rich in phosphorus.  These foods are organic sources of phosphate which are then broken down in the gut for absorption by the organism.  Excess phosphate is then excreted as waste in the form of inorganic phosphate which is what you will be testing for in your aquarium.

Just about any organic substance can break down and release phosphate into your aquarium.  Fish waste, coral slime, leftover food, detritus and dead algae cells are all very common and natural sources of phosphorus in your aquarium.

Even your tap water is also another potential source of phosphate. Underground minerals leach phosphate into most water supplies. Water treatment plants often add phosphates to protect pipes against corrosion.

This is one of the major reasons we really stress the importance of an RO/DI system at home. They remove harmful impurities, including phosphate, so you can be sure your source water is clean and free of contaminants. Just like nitrate, phosphate will build up in the aquarium water if left unchecked.

The good news is phosphate is not toxic and it won’t directly harm anything in your tank. Scientific studies of reef rock and corals reveal some interesting facts about phosphate. Here’s what we know so far. Phosphate is one of 17 essential elements that algae need to flourish. The general thought in aquariums is that phosphates fuel nuisance algae growth. Time and time again we have witnessed nuisance algae problems being resolved by simply reducing phosphates to a point at which the algae growth is limited.

On the other side, there are documented accounts of thriving reef tanks that consistently test with phosphate levels above the recommended range. The likely reason here is that in these reef tanks, phosphate is not the limiting growth factor for algae. Instead, the algae is being limited by lack of some other essential element.

The effects of phosphates on coral growth is also something that is important to understand when keeping a reef tank. The science suggests that phosphate is not directly toxic to coral. But, when phosphates are kept at higher concentrations, it can have some negative effects on coral. Symbiotic Zooxanthellae within coral are stimulated by phosphate, providing more energy for tissue growth. If phosphates are high for long periods of time, the zooxanthellae can overpopulate and turn the coral brown.

Science has also documented that the density of a stony coral skeletal mass is reduced when phosphate is kept at levels above 0.05 ppm. The belief is that phosphate inhibits the calcification process.

How Much Phosphate Is Too Much?

As hobbyists, we can deduce that phosphate is essential to your reef tank and recommended levels for a reef tank is heavily debated and has most certainly evolved over the years. For most reef tanks that are no more than 2-3 years old, phosphate levels should be kept very low in order to avoid problems. You’ll find recommendations ranging from 0.02 to 0.05 ppm are the most common answer.

On the flip side, we’ve all seen real-life reef tanks thriving at levels much higher than this at 0.1 ppm or more. In most instances, these tanks with higher phosphate are much more mature with larger coral colonies and a well diversified biological filter.

So, the real answer is – it just depends, every tank is different. I know you hate that answer, but let me explain.If your corals are consistently growing and nuisance algae are under control…the phosphate level is OK for your aquarium. If you’ve got algae problems, lower the phosphates and see how the tank responds. The idea is to bring the level down to where algae growth is inhibited and corals are still thriving, then maintain the phosphate at that level.

How To Test For Phosphate

The most convenient way to keep track of phosphate is with an aquarium test kit that measures the inorganic phosphate level of our tank water. Not all test kits are created equal so be sure to check out the range, resolution and accuracy of the phosphate test kit you choose. This will ensure that the test kit can give you the readings you are looking for.

The Hanna Instruments Phosphate checkers are an excellent choice we stock a few different models to accommodate different tanks and situations. The standard low range phosphate checker was designed for soft coral, LPS, FOWLR, or simply tanks that run with higher nutruent (nitrate) levels. It can measure phosphate levels from 0 to 2.5PPM with an accuracy of plus or minus .04ppm.

The Ultra Low Range phosphate checker is designed to provide more precision readings within the lower range that is typical for mixed reef tanks and SPS aquariums. The Ultra Low Range or ULR will measure from 0 to .9 ppm with an accuracy of plus or minus .02 ppm.

Then you have the Phosphorous ULR which is very similar to the Ultra Low Range Phosphate but instead, it measures in PPB - Parts per billion for an even tighter resolution in that low range. Perfect for those reef tanks running ultra-low nutrients where phosphate levels are not detectable otherwise.

Another option for testing is using one of the very popular mail-in or ICP water testing services that offer a phosphate or phosphorus test. ICP testing like this is accurate and gives you a comprehensive anaylsis of your tank, but it takes much longer to get results and is more expensive.

How To Control Phosphate?

In order to control phosphates in your reef tank, we aquarists have a variety of effective methods and sometimes it takes a combination of techniques to be effective. Generally speaking, we will not be able to rely on water changes alone so additional steps must be taken to target phosphate specifically.

First and foremost, good feeding habits and your bioload are critical. Overfeeding is the #1 cause of phosphate issues and if your tank as more fish than your filtration can handle, phosphate and nitrates will quickly become an issue.

In most tanks, phosphates are added at a much faster rate than what a regular water change schedule can remove effectively. Even a weekly 20% water change will not suffice in keeping phosphates below 0.05 ppm for a majority of reef tanks that contain fish. The fish simply require too much food to thrive so alternative phosphate control methods must be employed.

Protein Skimmer

Using a good protein skimmer will strip out organics, before they can release phosphate into the water. An efficient skimmer will remove dissolved and particulate organic matter, improving water quality and limiting phosphates and nitrates.

Refugiums and Algae Scrubbers

An algae reactor/scrubber or refugium with macro-algae are both excellent options for phosphate control as well.

A refugium with macroalgae will remove phosphate, nitrate and other nutrients via the fast-growing macroalgae. Chaetomorpha is the most popular and safest species of macro algae but Caulerpa and Gracilaria are also very popular options. Macro-algae grows very fast so you can oftentimes easily find other local hobbyists who are willing to share. Join your local reef club or hop on your favorite online forum such as Reef2Reef to meet other hobbyists in your area.

Algae scrubbers work on a similar principal but instead of macro-algae, you are growing turf or microalgae on a screen. Tank Water flows over an illuminated screen that holds the turf algae, and just like a refugium with macro-algae, harvesting the turf algae from your scrubber is very important. This physically removes the absorbed phosphates and keep the algae growing aggressively.

Filter Media

There are a variety of phosphate removing filter media that work in reef aquariums as well.

The most popular is Granular ferric oxide or “GFO” for short. GFO binds inorganic phosphate as water flows through the media. GFO works slowly when compared to other medias and is best used inside a fluidized media reactor to maximize media exposure.

Seachem Phosbond is another option that employs fast-acting aluminum oxide alongside GFO to absorb phosphates. Aluminuum oxide is fast-acting and is best used passively, inside a media bag or canister filter.

Blue Life USA just released Phos FX which is a new resin-based phosphate removal media. It is getting excellent reviews online because it can be regenerated and used over and over again. It is fast-acting and also best used inside a media bag. If using a media reactor, just be sure the media is enclosed in a media bag so as not to spread the tiny resin beads throughout your tank.

Liquid Phosphate Removers

Blue Life Phosphate RX and Brightwell Aquatics PhosphatE utilize Lanthanum Chloride which is another option and comes in the form of a liquid which is dosed directly into your tank. The lanthanum binds with phosphate, forming microscopic insoluble lanthanum phosphate particles. The inert particles are then removed via mechanical filtration and protein skimming. It causes a little cloudiness but works instantly. You will need to change out your filter socks and empty your skimmer cup a few times during treatment to be most effective

A clever and unique approach used by public aquariums is using GFO or similar media alongside lanthanum chloride. The lanthanum chloride liquid can be used to drop your initially high levels of phosphate to an acceptable range and the media can then be employed to keep it within this range thereafter.

This brings up a good point about phosphate control in general. A drastic drop in phosphate will really stress out corals and invertebrates. An aquarium that is accustomed to higher phosphate levels is especially sensitive. So be sure to test your water carefully and drop your phosphate levels slowly, over time.

Carbon Dosing & BioPellets

In situations where both elevated nitrate and phosphate are a problem, you might consider using something that is designed to target both. A product like Red Sea’s NO3:PO4-X Algae Management Supplement works by stimulating bacteria to consume phosphate and nitrates. The bacteria, then packed with phosphate and nitrate, are removed from the water through protein skimming.

Biopellets operate in the same fashion but instead of a liquid to fuel the extra bacterial activity, you are using a natural corn-based plastic media that is tumbled inside a media reactor. Once established, bacteria grows and consumes these pellets and also takes in nitrates and some phosphate along with it. The biomass of nutrient-rich bacteria is then removed from your tank via protein skimming.

Choosing the right phosphate control methods should be aimed specifically at your tank’s needs. The first priority for everyone is to limit the input of phosphate to begin with. Get an RO/DI unit and only use phosphate-free water for top-off and water changes. Don’t over-feed your tank and be sure to turn off your pumps when feeding. Use a quality protein skimmer at all times to remove organics before they break down into phosphate. If you have the space, add a natural filter like a refugium or turf algae scrubber.

When phosphate levels are continually problematic, only then would I recommend the regular use of a GFO, Phosbond, and lanthanum chloride based liquid phosphate removers. In this case, I would also recommend you reduce your stocking levels or improve your filtration system to help maintain a more acceptable level of phosphate long term. Relying on chemicals or media at all times can be a slippery slope and will get expensive.

The main point to understand about phosphate control is that you should not be chasing a number, rather, a clean and healthy reef.

When your tank surfaces and rocks are clean and algae-free, your phosphate level is likely safe and should only be monitored alongside all of your other major water parameters. If you start to see nuisance algae growing in your tank or notice your corals are just not looking their best, start testing your phosphate levels more often. If you notice rising or elevated levels above 0.05 ppm, you should then begin working on removing phosphate and establishing a system to maintain it at a lower level.

To learn more, check out our video series Master Nutrients: Nitrate, Phosphate and Reef Aquariums for a deep dive into phosphate and nitrate control in a reef aquarium.