Phosphates and Phosphate Adsorbing Media

Where Do Phosphates Come From?

Here are some possible sources for you to investigate if you have elevated phosphates (PO4) in your saltwater aquarium:

  • Prepared fish food in the form of pellets or flake food is a major contributor to phosphate levels. Even the juice from thawed frozen foods can elevate phosphates.
  • Tap water is another source of phosphate which is why only Reverse Osmosis (RO) or deionized (DI) water should be used in our saltwater aquariums.
  • Phosphates are often bound to dead rock that had not been properly scrubbed and rinsed before being introduced into the aquarium. Over time, the bound phosphate is gradually released into the water, causing high phosphate levels in the long term.
  • Phosphates are also leached from activated carbon which we commonly add to our aquarium filtration systems to clarify water.  Although the amount leached is very small, phosphates accumulate when there is a lack of regular monthly water changes.

How Is Excess Phosphate Dangerous?

While phosphates are utilised by corals in very small amounts for tissue regeneration, excess phosphate stops their growth and eventually kills them.

Zooxanthallae — the important symbiotic algae in the tissue of corals and clams — multiply too quickly when there is too much phosphate. This causes the coral to eject its zooxanthallae, resulting in coral bleaching. While bleaching is sometimes reversible for a coral in captivity in our aquarium, coral bleaching often results in death.

Phosphates – How Much Is Too Much?

In a full-blown reef aquarium with many corals, we should aim for as close to zero phosphates as possible, with 0.03ppm (parts per million) of phosphate (PO4) being an acceptable level. At as little as 0.25ppm, coral tissue starts to recede.

Although phosphates at elevated levels in the aquarium seem to have no effect on fish.

Phosphates And Algae Growth

Excess phosphate is also a major cause for algae blooms in our aquarium.  Just like weeds in a garden, algae can grow uncontrollably with high phosphates acting as a fertilizer.

While a small amount of algae is welcome, especially if we have herbivorous, grazing fish such as tangs and rabbitfish, we do not want algae to overrun our saltwater aquarium. Too much algae is unsightly at best. At worst, it covers live rock like a carpet, smothering corals and eventually killing them.

Reducing Phosphates In The Saltwater Aquarium

There are a several methods by which we can reduce phosphate in our aquariums:

  • Do not overfeed the fish. Simple enough, but ensuring that every bit of food is consumed is a good way to ensure that phosphate levels do not creep up.
  • Stepping up the frequency of water changes is effective when trying to reduce slightly elevated phosphate levels in most cases. If you do not see a drop in phosphates after performing 10% weekly water changes over the course of one month, then phosphates are being leached from the rock in your aquarium. It will take many discouraging months of frequent water changes before you see a considerable drop in phosphates. Even then, phosphate readings may remain stubbornly high despite your best efforts.
  • Using phosphate remover which is by far the most effective method for reducing phosphates and keeping it at near zero levels.

Types Of Phosphate Adsorbing Media

There are 2 main types of media available to the aquarist — aluminum oxide and ferric oxide (sometimes referred to as GFO or granular ferric oxide).

There is some evidence that aluminum oxide releases alumina into the water, causing an adverse reaction in some corals. This has been my experience with the use of aluminum oxide media.

At recommended amounts, I find that ferric oxide media is completely coral friendly while being extremely efficient at controlling phosphates.

As far as GFO’s, I have used both PhosBan and Rowa Phos. Both work equally well, but my money goes with Rowa Phos.

Fresh Rowa Phos is a dark rust color — and rust is actually what it is. It is also moist and needs to be kept well sealed in its container to prevent drying out which would lower its efficacy.

Phosphate Adsorbing Media In A Fluidised Reactor (FR)

While we could run GFO passively in a bag placed in medium current in the aquarium sump, utilizing a fluidised reactor will allow us to get the most out of it. We also want to maintain a low flowrate going into the reactor so as not to over-fluidise the media as we want to maximize its contact time with the aquarium water.

Two Little Fishies’ PhosBan Phosphate Reactor works extremely well for this purpose.  It also comes with an adjustable valve so you can set the ideal water flow entering the reactor chamber from the FR pump.

I normally adjust the valve so that the GFO remains at the bottom of the reactor chamber and is ‘tumbling’ gently, which is ideal. Over fluidising the media may also cause fine brown particles of GFO to escape the reactor and escape into the aquarium. While not harmful to corals or fishes, it can be unsightly if it were to cover the substrate.

The aquarium water immediately exiting the reactor chamber should read at zero when tested for phosphates.

Changing Phosphate Adsorbing Media In A Fluidised Reactor

When changing the media in my fluidised reactor, I spoon the appropriate amount of fresh GFO into the reactor chamber without pre-rinsing it first. Sealing the reactor and connecting the pump, I let the first couple of gallons of aquarium water run through the chamber, out from the FR outlet and into a bucket. The water is running brown at this point from the fine GFO particles and we do not want this in our aquarium.

Once the water running into the bucket starts to run clear, I place the FR outlet back into the sump. I then top up the same quantity of saltwater lost with new saltwater prepared beforehand just for this purpose. Every change of GFO media is also a small water change for the aquarium. Always a good thing.

Using Phosphate Remover During Tank Cycling

To eliminate any future problems with elevated phosphates, I prefer to run GFO in a fluidised reactor right from the start. During the cycling phase of starting up a new aquarium, phosphates will be leached from live rock as tiny organisms within the rock die off. As mentioned in the introduction, using dead rock is one of the causes for long-term elevated phosphate levels. GFO in a fluidised reactor will adsorb any phosphate as soon as it appears.

Using Phosphate Remover On A Permanent Basis

Just like running a protein skimmer, we should run phosphate removing media in a fluidised reactor on a permanent basis. The amount to use depends on the size of the aquarium and the manufacturer’s instructions, but using up to 3 times the amount usually causes no ill effects other than perhaps an initial slight depression in pH which should then be buffered upwards.

GFO should be changed when levels reach 0.05ppm measured on a reliable test kit.

What If Phosphates Are Still High Despite Using Phosphate Remover?

This is actually a very common scenario, especially if phosphate remover was not used during the cycling phase — phosphate bound in cured, uncured or dead rock leaches back into the water slowly, and no matter what we do phosphates remain high.

The solution would be to use up to 3 times as much GFO as suggested by the manufacturer and monitor phosphate levels closely.  If you use enough GFO in your phosphate reactor, you should be getting zero readings of phosphate in the water exiting the reactor. Once levels exiting the reactor become measurable on a test kit it is time to change the GFO media.

This can take several changes of large amounts of GFO during the course of about a week or two, so be prepared to invest in a a fair amount of media.  Your care and investment will pay off once phosphate levels reach zero in your aquarium, from which point you can run the normal recommended dosage for your tank size.

I would NOT recommend high dosages of aluminum oxide-based phosphate remover because of the undesirable impact it will have on corals in your tank!

And always monitor pH levels when running large amounts of GFO!

Controlling phosphates continual basis is one of the key elements to maintaining a beautiful saltwater aquarium.

Read our PhosBan Reactor 150 Product Review

Links to more pages on Water Parameters:

Ideal Water Parameters for a Reef Aquarium

pH and Alkalinity

Ammonia and Nitrite

Nitrate in the Reef Aquarium

Calcium in the Reef Tank

New Tank Syndrome

Water Parameters | Danger Levels 

Links to more Saltwater Aquarium Filtration pages:

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