Phosphorus is a vital macronutrient that all living cells require. However, elemental Phosphorus is so strongly reactive that it doesn't normally appear in its unoxidized form in nature, but always as phosphate, in which the P atom is surrounded by four oxygen atoms (PO₄). Phosphates are part of DNA and RNA; phosphate is used in photosynthesis; in blood, a phosphate buffer keeps pH stable; phospholipids are the major components of cell membranes. Phosphate also participates in every living cell as the P in ATP (adenosine triphosphate), the basic "energy currency" molecule that all life uses to transfer energy. In short, phosphates are everywhere.

Phosphate phases. Phosphates cycle naturally both in organic forms, called "organophosphates," where PO₄ groups form part of organic molecules, and in inorganic, mineralized forms, called orthophosphates. These can string together in chains called polyphosphates. And phosphate groups of either kind may be dissolved in the water or exist as particulates, whether precipitated out or adsorbed to a substrate. Only phosphates that are already built into organic molecules can be used by animals, including fish. Only the soluble orthophosphates are available to algae and plants: the biologists who study freshwater systems term them "soluble reactive phosphates" (SRP) because the insoluble orthophosphates are unavailable to organisms in that phase, thus "unreactive." Your phosphate test kit is measuring only this soluble inorganic phosphate. To measure the organic phosphates you'd have to boil your sample in acid, topping up repeatedly with distilled water, to break down the organic PO₄ to SRP. Not for the aquarist's home kitchen. And bacterial action is constantly tapping the aquarium's reservoirs of organic phosphate, mineralizing it to SRP. So your test result might be different tomorrow.

Some phosphate in food is metabolized and deposited with calcium in fish bones, teeth and scales, mineralized as calcium phosphate, or apatite. You could think of these organic forms of PO₄ as insoluble too-- locked up and insoluble, like the phosphates locked up in living plankton.

Animals aren't able to take up inorganic phosphates, even if they are dissolved. Instead, soluble reactive phosphate is scavenged by bacteria and algae and also quickly taken up and used or stored by plants. The "higher" plants have this ability to store PO₄, so in a system where plants are in control, living plants form a major reservoir of the system's phosphate. Once phosphates are stored in plant tissue they are no longer available to bacteria and algae. Instead, grazers or detritivores assimilate the organic phosphates, which get passed up the food web. What the zooplankton or the fish can't use, they excrete in their feces, where gut bacteria have already begun reworking it. So, when plants or animals die, bacterial decomposition remineralizes their phosphates. This cycling moves fast, so fast that unpolluted natural waters are normally extremely low in phosphates, which may measure in parts per billion. But algae can get by on a PO₄ diet of 1 to 10 ppb. Not easily phosphate-starved.

In deep lakes, the productivity of freshwater algae is limited by the availability of scarce phosphate. British experiments in Lake Windemere confirmed this. When phosphates were stripped from the wastewater and run-off entering the lake after 1992, the concentration of blanketweed, a cladophora alga, was reduced.
What happens in deep lakes is that both phytoplankton and the planktonic grazers that have scavenged the available phosphate, eventually sink to the bottom far below, taking the phosphate out of circulation. In aquaria, by contrast, phosphates are generally over-plentiful, and they are credited with algal blooms. Though perhaps phosphate and algae are too rashly linked, many of us are still interested in lowering phosphate levels.


Sources of phosphate. Animal feces contain significant amounts of phosphate. Think of seabird or bat guano. The accumulated droppings of fish-eating seabirds are so rich in the phosphates that have passed through their systems, that guano forms the main mineral wealth of some oceanic islands. Similarly, the major source of phosphate in the aquarium comes from the fish meal content of flake feed, where the ground-up fish of "fish meal" includes the calcium phosphate (apatite) of bones and scales. Though it provides healthy roughage, calcium phosphate is indigestible in this form and passes out in fish feces: "fish guano." Recently some "lowered-phosphate" flake feeds have come onto the market to answer fishkeepers' concerns about phosphate levels. In the aquarium, organic phosphates remain in plant detritus as well as fish feces. Bacterial degradation will swiftly reduce them to algae-available orthophosphate. Though the vascular plants tend to take up PO₄ before algae can use it, this is a major incentive for you to siphon out the detritus and rinse your filter!

Unnecessary phosphate can enter the aquarium's closed system through other paths. Phosphate may enter the system in many water conditioners, and some common pH buffers still contain phosphate. In the past, it also used to be released from inappropriate forms of granular activated carbon. Some houseplant fertilizers that have been re-labelled (but not reformulated) for aquarium use will contain phosphate-- it's the "P" of familiar N-K-P, after all. There's phosphate in the rock-wool that wraps the roots of your new hydroponically-grown plants, if you haven't snipped open their plastic baskets and picked it all away.

It's easy enough to eliminate these sources of phosphate in your tanks.


Tapwater polyphosphates. It's not so easy to eliminate the phosphates that enter your system in your tapwater. Long-chain polyphosphates are often added to the public water supply, both for protecting water mains from corrosion and carbonate scale and also for some softening effect, when the PO₄ renders calcium or magnesium ions inactive by sequestering them. These forms of PO₄ are orthophosphates, which are available for plant and algal uptake. (Protecting these useful added phosphates is the reason it's illegal to dump FeCl in your drain.)

Phosphate burial. Phosphate carries strong triple negative charges--— and so do colloidal clay and humus particles. But the clay and humus attract positively-charged cations, which cluster round, and the negatively-charged phosphates bind to those. Heavy PO₄ loading in the water can contribute to persistent initial cloudiness caused by colloidal clay, as the not-yet-neutralized negative charges repel one another, keeping the charged colloidal silt from settling out. In aquaria this can be an issue in a newly established aquarium where laterite is part of the substrate mix and phosphate-based buffers are forcing down the pH. Similarly, in wastewater treatment plants, high levels of phosphate can interfere with coagulation in settling tanks. The phosphates adhered to the colloids can give the colloids net negative charges that encourage them to repel each other and stay dispersed in the water.

But natural reactions are constantly pulling phosphate from the water. For example, iron is quickly oxidized in oxygen-rich waters, and the resulting positively-charged iron hydroxides scavenge phosphate from the water, bind it, and carry it as a component of ferric precipitates, which form on the substrate. In a moment I'll note how this reaction can be harnessed to reduce phosphate in the water. In highly buffered alkaline waters, however, phosphates tend to bind directly to calcium instead. So, with one kind of partner or another, phosphates are continually being extracted from the water and buried in the substrate--— as long as the substrate contains some colloidal clay and flocs of humus, or some suitable iron or calcium compounds. This puts soluble reactive phosphate in the substrate, where plant roots can get at it, but where it's not available to algae. "Indeed," says Diana Walstad, who's recently explained these things, "if a soil sample is shaken with a concentrated phosphate solution, it will remove the phosphate." In a natural environment, where phosphates are bound in sediments, many plants prefer to take them up by their ordinary root respiration. In a tank with no rooted plants, you'll quickly see how phosphate could accumulate in the sediment, safely taken out of circulation, until you stir it up with a gravel vacuum. Part of what makes a substrate "age" is the accumulation of phosphate; sometimes long-established aquaria slowly develop intractable algae problems, and the phosphates in the substrate are part of the problem.

Thoroughly washed conventional "aquarium" gravel has few appropriate sites to hold phosphates, for clay and humus are not elements found in ordinary "aquarium" gravel. Yet any orthophosphates that remain in the water column encourage the growth of single-cell algae. If you sense, as I do, that there's a connection between plain gravel substrates and problems with algae and green water, lack of a natural phosphate sink might be the invisible link. Some colloidal silt, such as laterites offer, mixed into the original substrate, may be contributing to low phosphate levels in the aquarium water.


Precipitating phosphate with iron. To reduce PO₄ levels in the water, some advanced aquarists have been adding iron, in the form of ferric chloride (FeCl), an ingredient, it would appear, in Tetra's EasyBalance. The iron reacts with phosphate to form insoluble iron phosphate, which precipitates out. These avant-garde types were inspired by Peter Peterson's translation, in Aquatic Gardener, vol. 7, no 1, of a German article from AquaPlanta. The technique is to slowly reduce PO₄ by adding a 0.3% solution of FeCl over the course of several days. Paul Sears and Neil Frank discussed this in the Aquatic-Plants Digest, 4 Apr 1996 etc. If you do decide to use FeCl, make sure it all goes eventually into the aquarium, for it's against federal law to dispose of it down the drain.

Luca, posting in the Aquatic-Plants Digest, 3 Feb 1999, gave the operative chemical reactions (it's my translation):

FeCl3 + 3.H20 -> Fe(OH)3 + 3.HCl
"Ferric chloride reacts with 3 molecules of water to give ferric hydroxide and 3 molecules of hydrochloric acid."
FeCl3 + Na3PO4 -> FePO4 + 3.NaCl
"And ferric chloride also reacts with sodium phosphate to give iron phosphate and 3 molecules of common salt."
2.HCl + Ca(HCO3)2 -> CaCl2 + H2O +2.CO2
"Then two molecules of the hydrochloric acid react with two of calcium carbonate [from the buffer] to give one molecule of calcium chloride, a molecule of water and two of carbon dioxide."


Precipitating phosphates with alum. Alum (aluminum sulfate AlSO₄) is a flocculant used as a water clarifier in many public utilities. It does not affect the alkalinity. Dissolved in water, it forms a floc (aluminum hydroxide, the main ingredient of Maalox) that may temporarily cloud the water while it reacts strongly with dissolved phosphate, precipitating it out as insoluble aluminum phosphate. Aquarists and pondowners who are bent on controlling algae by reducing PO₄ may want to use AlSO₄ to flocculate PO₄ and collect phytoplankton as it settles out.

Sweetwater Technology Corp. of Aitkin MN outlines the water chemistry involving alum in an article "What is alum and how does it control algae?" in ponds.


Phosphate-reducing granules. Resin granules that adsorb phosphate can be included in your filtration. Tim Hovanec downplays the effectivenesss of chemical PO₄-reducing granules or pads as too slow-acting to be effective and also apt to be quickly colonized by the bacteria that normally re-solubilize the adsorbed phosphate. Healthy growing plants and good sanitation practices go farther than chemical filtration media to keep phosphate levels low. Wastewater management engineers find that the bacteria in activated sewage sludge also have a high tendency to absorb phosphates from wastewater. So when you rinse the filter media, you remove organophosphate before bacterial action can mineralize it to orthophosphate.

Re-releasing adsorbed phosphate. In the aquarium, phosphate adsorbed to colloidal silt and floc becomes soluble again in anoxic underlayers of your substrate, where it is available to plant roots. If you vigorously stir up your substrate at intervals, this soluble PO₄ is released into the water column temporarily. Before it precipitates out once more, it is available to algae. This is a cause of frustration for aquarists whose gravel-cleaning and general sanitation practices are meticulous and yet are plagued by cloudiness and green water.

A shortcut in phosphate cycling. Phosphates are only recycled on the planetary scale in vast slow biogeochemical processes that take tens of millions of years. In the shorter run, all the earth's phosphates are on a slow ecological conveyor belt that finally deposits them on the oceans' abyssal plains,. There tectonic seafloor spreading will make them available again to a future age. In a closed ecosystem, such as an aquarium, phosphate must be continually reintroduced, and then it is transmuted back and forth between its organophosphate and orthophosphate forms, until it's eventually lost through water changes, filter rinsing or plant prunings --or the removal of a dead fish. Unlike nitrate, which is constantly generated in the aquarium ecosystem from ammonia, phosphate can't be generated through biological processes. Also, there's no store of PO₄ in the atmosphere to draw on, no gas phase for phosphorus as there is for nitrogen.


The enzymes called phosphatase. Within aquatic ecosystems, continual regeneration of orthophosphate from organic phosphates is performed by a range of enzymes collectively called phosphatase. These enzymes are the keys to access otherwise unavailable resources of organic phosphate. The process is rapid: turnover rates for scavenged phosphate are measured in matters of minutes and hours. Though many aquatic organisms, ranging from bacteria and cyanobacteria to algae and even higher plants, can synthesize these enzymes, the two major sources of phosphatases are algae and aerobic bacteria. Anoxic conditions inhibit the production and activity of phosphatase: thus the anoxic layers of an unplanted substrate can become a phosphate sump. Whenever local levels of orthophosphate drop low, bacterial communities in the biofilm draw on phosphate bound in the sediment: in response to low availability of free inorganic PO₄, synthesis of phosphatase will switch on within 24 hours or less. Conversely, sufficient supplies of orthophosphate tend to repress the production of phosphatase.

Accumulating organic matter allows an increase in biofilm bacterial populations. More bacteria exert more demand for phosphorus. The less orthophosphate is available, the more necessary local recycling becomes, and the more freely phosphatase is produced. So a kind of stabilizing feedback system is established, which is characteristic of aquatic systems as a whole --and perhaps characteristic of the greater aquarium that is Gaia.

You knew that vacuuming out the mulm and decaying plants would help control PO₄ levels: now you know why.


Cyanobacteria and phosphate? In marine ecosystems, cyanobacteria have a much higher rate of phosphatase production than algae. "This enhanced enzyme gives cyanobacteria an advantage over algae in P-limited conditions in the presence of labile organic matter [in Florida Bay]" one observer noted. Perhaps there is a clue here to the blooms of cyanobacteria we sometimes encounter in freshly set-up freshwater aquaria also. The sheets of cyanobacteria seem to subside as algae and the biofilm become established, and as PO₄ begins to accumulate in sediments. In our freshwater aquaria, PO₄ is rarely limited, except in newly-set up systems. Could cyanobacteria have a similar phosphate-scavenging advantage in freshwater aquaria?


Phosphate links. Wilkes University has a phosphate website where these questions are explored.

At the Aquarium Frontiers site, an archived article on phosphate by Randy Holmes-Farley, "Phosphate...what is it and why you should care," escaped me for a long time, because it was "about" marine aquaria and didn't seem to apply to me. I found there's a lot here for the freshwater fishkeeper too. He discusses inorganic and organic phosphates, sources of phosphate in fish food and tapwater and phosphate sinks. At the high pH of seawater, PO₄ precipitates onto calcium and magnesium carbonates, minerals that are in short supply in my soft water, and in your water too perhaps, unless you're maintaining an aquarium for Lake Tanganyika's alkali-loving cichlids.