How to Control Phosphate in a Saltwater Tank

control phosphates in a saltwater tank

Phosphorous is present is every reef aquarium.  From the moment you cycle your tank, your relationship with phosphate will develop over the years. Learning to control it from the beginning will reap huge rewards for the future of your aquarium. Phosphorous is present in every form of life. Our aim is to keep phosphate levels low enough in a marine tank to ensure it does not cause unwanted algae growth and inhibit the calcification of coral. In this article we cover the huge subject of how to control phosphate in a saltwater tank.

How did I get phosphate in my saltwater aquarium?

There are three main sources of phosphate introduction into a saltwater aquarium, all of which can be controlled.

1. Fish and coral food

Phosphate is introduced into your system mainly through feeding your fish and coral. You could be feeding flakes, pellets, or even frozen mysis. You may be broadcast-feeding the coral in your tank too. Any food addition to a saltwater aquarium is an addition of phosphate. This is the main reason, as a reefer, you need to consider the amount of food you are feeding your livestock. Are they consuming it in 20 to 30 seconds? Do they really need that much food morning and night?

If you are feeding your fish frozen food, it is highly recommended to thaw it in RO water. Not only will this aid in thawing the food quicker, but it also allows the food to be rinsed in the RO water, ridding it of excessive phosphates, before placing it in the water to control phosphates in a saltwater tank.

2. Water source

Another source of phosphate is the use of inadequately filtered water or tap water. Using tap water in a marine tank is thoroughly discouraged because of its potential to be a significant source of phosphate, among other contaminants. Instead, RODI water should be used. This has been passed through a filtration that results in pure, clean, phosphate free water, suitable for any marine tank.

Be sure to check your RODI water for phosphate if you purchase it from elsewhere, if you suspect it may be the culprit. This can sometimes occur if the filters in an RO unit have become exhausted and no longer provide the correct level of filtration necessary.

3. Phosphate bound live rock

More and more often, we hear of phosphate being bound in live rock in a new setup. As reef keeping has become more popular, so too has the rate in failure. These failures occur through lack of proper care and research, and invariably end with a tank being broken down and live rock being offered at a substantially reduced price. You will find that most tank failures come down to excessive nutrients (nitrate and phosphate) in the system, and the inability of the inexperience hobbyist to control them. Live rock which has been submerged in such a system will absorb the phosphate, and when removed, will have phosphate bound to it. By purchasing this cheap, phosphate-bound rock and placing it into a new system, you are effectively introducing a continuous source of phosphate. It will eventually leach out, but this can be a long process taking several months. Do not buy second-hand live rock if you want to control phosphate in a saltwater tank.

How to control phosphate in a saltwater tank

To control phosphate in a saltwater tank, phosphate media control is an important addition to any marine tank that should not be underestimated. With the number of different methods available to reduce phosphate levels and maintain them at lower levels, we really have no excuse. When considering phosphate control, the available absorption media can be split into three main categories

1. Aluminum oxide

Aluminium Oxide is extremely effective at phosphate removal to such an extent that it can strip a system if not used in the right manner. Instructions must be followed to ensure you do not overdose. It can be run in a fluidized reactor as long as there is no movement of media. Any movement can cause media to dust, which will find it’s way into the display. This has been known to cause problems with corals and fish. It is ideally suited to marine tanks without sumps. It can be placed in a bag in high flow areas (rear compartment, canister filter). The following media belong to this group:

Two Little Fishies Phosban

Seachem PhosGuard

The above media can be seen to be exhausted when the granules have turned from white to yellow.

Seachem PhosGuard 1 Liter
  • Country Of Origin : United States
  • The Package Height Of The Product Is 3.6 Inches
  • The Package Length Of The Product Is 7.9 Inches

2. GFO (Granular ferric oxide)

GFO can be a little easier on your system because it does not eliminate phosphate as aggressively as Aluminium oxide does, but it is just as good, if not better at controlling it. Once again instructions should be followed, but not due to an aggressive nature, but because overdosing is wasting your money. Using any more media than advised will not hasten the process, but it can make it more aggressive. GFO performs better when gently ‘tumbled’ in a fluidized reactor, and is hard in nature, therefore will not release dust into the display. Although GFO is ideal in a reactor, it can also be placed in a media bag placed in a fast moving flow in the sump. The following media belong to the GFO group:


TL Reefs GFO Phosphate Remover

The above media should be monitored at the reactor outlet. Should the reading at the outlet climb above 0.03ppm, it is time to replace the media. I have personally used Rowaphos and other GFO phosphate removers. All of which I have used in a Phosban 150 fluidized reactor connected to a 200gph pump. What I like about the reactor is that it is easy to maintain and clean, and is supplied with a control valve allowing you to control the flow into the reactor.

For more information on ROWAphos, read this article.

For more inormation on GFO’s, read this article.

D-D RP-50 ROWAphos Phosphate Removal Media - 500 ml/16.9 oz
  • Best Selling Phosphate Remover
  • Used Widely all over the world
  • Largest absorption ability

3. Lanthanum Chloride

Lanthanum Chloride is a relative newcomer to the phosphate fight, and unlike all the granular media on offer, this is a liquid dosing schedule. According the literature the Lanthanum Chloride acts as an ion exchanger and uses calcium to bind the phosphate into insoluble beads that are picked out by the filtration. As with all of the above, reports are positive, and many reefers choose to use this as their method of control. But it can be an aggressive phosphate remover. If phosphate is removed from the system too rapidly it can cause an undesirable bottom out. The following products belong to this group:

ATM Aquarium Products Agent Green

Blue Life Phosphate RX

Brightwell Aquatics Phosphat-E

Your choice of phosphate remover is completely up to you, based on your situation, and what you find easier to use. I have always used GFO’s, but that is not an endorsement of GFO’s. It is only what suits my situation.

Brightwell Aquatics Phosphat-E - Liquid Phosphate Remover for Marine Fish and Reef Aquarium 250-ML
  • Eliminates reactive phosphate immediately upon addition to aquarium
  • Safe for all inhabitants of reef and marine fish-only aquaria; May be used on an ongoing basis to help control phosphate in heavily-stocked aquaria
  • Each ml eliminates 1 ppm phosphate in 4 US-gallons of water

GFO is not reducing my phosphate

It will quickly become apparent on social media and forums that each reefer seems to have his or her preferred phosphate media, singing it’s praises while telling you how ineffective all others are. This can be very confusing for any new hobbyist, or indeed anyone thinking of changing their product.
The truth is that far too often any given media is used incorrectly. The user does not appreciate that if you have excessive phosphate in a system, whatever the media is, it will quickly become exhausted and need replacing. This can take a matter of days. It is important to realize that phosphate media can only last so long, and that it is your responsibility to monitor and change it over when it has expired.

When you find that your phosphate is not reducing, you can assume that the media is working, but that you have phosphate bound to your rocks. This can occur if you have not used phosphate media before, or if you have allowed your phosphate to increase unchecked.

To remedy the situation, you need to draw the phosphate from the rocks. You will have to replace your media every three days until you see a drop in phosphate levels. When you see a drop, ensure you bring it back down to between 0.03ppm and 0.05ppm. Once this level has been achieved you will only need to replace your phosphate absorption media every 4 – 6 weeks.

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Can phosphate absorbers strip corals?

Aluminum Oxides can be very aggressive and can strip corals due to their ability to quickly remove all phosphate from the system. To a lesser extent, GFO are also capable of doing this. Instructions should be followed as closely as possible to ensure no harm comes to your tank. Aluminum oxide also have the potential for micro particles to find their way into the display, but only through too high a flow in a fluidized reactor. If the correct low flow is used, no media will escape the reactor. There have been reports of Aluminum oxide breaking down into a dust within the reactor and finding it’s way into the display, however this is mainly due to overuse.


Just like nitrates, there is no overnight magical cure. It can be one of the most frustrating and infuriating aspects of reef keeping, and no matter who you talk to, everyone will have their own method, or multiple methods of control. Each system is unique in how it responds to your method of control. Be patient, put your method of control in from day one, right after the live rock goes in, and monitor it. Whatever you do, do NOT buy live rock for next to nothing from someone you don’t know who is breaking their tank down – phosphate will be bound to those rocks

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I am the owner of The Salty Side.
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How to Control Phosphate in a Saltwater Tank