Phosphate

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Water Parameter
Phosphate
Unit(s): ppm
Target: 0.02 ppm
Danger: > 0.5 ppm
Tendancy: to increase
Increased: Food

Unrefined Tap Water

Decreased: Algae

Water changes Phosphate Binder

Testing: Periodically
Maintenence: Water changes

Protein skimmer Phosphate Binder Carbon Dosing Calcium hydroxide


Introduction

Phosphate is an important compound that is a key component used in all living things, in cellular metabolism it forms an integral part of energy transfer in the form of ATP (Adenosine Triphosphate). However, due to the fact that it can be a limiting nutrient for algae growth and it can inhibit calcification, it is best to keep it to a level as low as possible.

Sources

There are three important sources of phosphate that can introduce it into a system.

Food

Food is the primary source of phosphate for a reef aquarium. All food contains some phosphate in it, since it is an important component of all living things. The addition of one cube of frozen mysis shrimp to a 400L aquarium in a day will add approximately 0.03ppm PO4 to the water. A feeding of a similar amount of protein in the form of a dried food will add slightly more.

Top Up Water

Water used to replace the water lost by evaporation can end up being a significant source as it is added to the system constantly. That is why it is not recommended to use raw tap water, it should be filtered by either RO or DI (preferably both) before use.

Water Changes

When performing a water change, the new water can include phosphate.

Testing

Colourmetric Test kits are commonly available, however their accuracy and resolution is limited at best within the ranges pertinent to Reef Aquariums, digital phosphate meters provide greater accuracy and resolution more attuned to the requirements of Reef Aquarists. Most aquarium phosphate testing is limited to inorganic forms of phosphate.

Control

Methods that can be used to control the amount of phosphate within a system are:

Precipitation

Precipitation of calcium phosphate is favoured at high pH (typically, pH ≥ 8.4), such as present in the regions where kalkwasser is dosed.

Algal Export

Phosphate is an essential component of algal growth, most famously noted by the redfield ratio (C:N:P = 106:16:1), this can be exploited through the controlled growth of algae. A favoured method is to either use micro algae (such as in an algae turf scrubber) or macro algae (in a refugium ) then regularly harvest the algae.

Protein Skimming

A protein skimmer removes organic material and living organisms from the system, thereby exporting organic forms of phosphate. Inorganic Phosphate will not bind with air and will not be skimmed.

Binding

Some materials can actively bind phosphate to them, such as aluminium oxide, ferric oxide / ferric hydroxide and salts of Lanthanum. Both aluminium oxide and ferric oxide/hydroxide are granular in form and remove phosphate from the system permanently when replenished, Lanthanum salts act as a flocculant and should be either skimmed or physically removed from the system through the use of a filter sock. Under certain circumstances, phosphate may be released from the medias back into the water column, however with regular replenishment of the media this is expected to be limited.

Bacterial Export

Induced with carbon dosing and combined with skimming, bacteria have the ability to take up phosphate along with nitrate, similar to the redfield ratio noted earlier, through cellular metabolism and provide an export means through skimming the bacterial mass.

Resources

References