To take on the responsibility of maintaining a marine aquarium is to take a crash course in water chemistry. By the time you’ve successfully navigated the nitrogen cycle, you’re already faced with new, threatening chemical imbalances. Of these, one of the most persistent (though frequently unnoticed) is excessive phosphate.
Perhaps the best way to start a conversation with aquarists about phosphate is to state its ecological importance. Phosphate is good, necessary even. Indeed, no known organism can survive without it. In other words, you cannot operate an aquarium for very long without it. So why is it treated like a deadly poison by some aquarists?
Too Much of a Good Thing
Being as phosphate is only minimally present in the air, the oceans take in very little phosphorus from the atmosphere. Phosphorus is rather transported to marine environments primarily by way of rivers. It is therefore more concentrated in coastal than in open ocean environments. It may exist as particulate organic phosphorus (e.g. detritus), dissolved organic phosphate (e.g. those embedded in certain fats and proteins) or dissolved inorganic phosphorus (e.g. phosphate).
For the most part, as soon as phosphorus enters a marine ecosystem it is taken up by primary producers (mainly phytoplankton). This pathway in the cycle, known as the “biological pump,” delivers phosphorus locked up in biomass (carcasses, feces and so on) to the seafloor where it becomes part of the detritus pool. As detritivores consume and metabolize the material, they release phosphate into the surrounding waters. This inorganic phosphorus is returned by up-welling, oceanic water currents, etc. to the surface where it can again be assimilated by algae and other autotrophs.
Though it is vital, phosphate is generally required only in minute quantities. This creates serious problems in recirculating aquarium systems, where the rate of input can greatly exceed the sequestration/export rate. This elevates phosphate concentration far above optimal levels.
Phosphate concentrations below 0.2 parts per million (ppm) are recommended for most reef systems. Nuisance algal growth usually becomes evident as phosphate concentrations exceed one or two ppm. Yet some poorly maintained systems can reach levels as high as 5.0 ppm or more. At these super levels, not only can bad algae grow at terrifyingly high rates; calcification is impeded in stony corals, very much adversely affecting their health.
Saying Goodbye to Excess Phosphate
Modern technology and advanced filtration/water treatment techniques are able to maintain extremely low phosphate levels. Still, it is quite rare for one to need to add phosphate directly to a reef tank. Far more often, reef aquaria are loaded with the stuff.
There are a number of ways that one can limit the introduction of phosphorus into the system. These include:
- Avoiding use of untreated (i.e. not purified) tap water.
- Not overstocking livestock.
- Limiting amount food input.
- Using and regularly replacing special chemical filtrants.
- Consistently and frequently carrying out water changes.
- Installing a planted refugia.
- Carbon dosing.
The surest way to remove phosphates (assuming purified freshwater is used to make the new saltwater) is old-fashioned aquarium water changes using purified make-up water. The same should be used for evaporation top-off. Even if you have installed top-of-the-line aquarium filtration equipment, regular water exchange is necessary for the long-term health of the system. The most efficient way to get rid of phosphate is to vacuum the substrate when removing old water. As this removes much more detritus, it helps to prevent the accumulation of phosphorus in settled particulate organic matter.
A planted refugium is a very natural way to control nutrients including phosphate. The macroalgae assimilate phosphate and other nutrients as they grow. These nutrients are exported as portions of the algal mass are harvested and thrown out. Detritivorous microcrustaceans, such as copepods, heavily populate refugia; therefore, this method additionally helps to release phosphorus from detritus into phosphate, continuously fertilizing the macroalgae and preventing the stockpiling of phosphorus in settled particulates.
There are chemical filtrants that absorb and hold onto phosphate. Both ferric iron granules and aluminum oxide beads are used pretty effectively for this purpose. Ferric iron is an especially strong phosphate scavenger but is prone to compaction/clumping; whenever used, it must be contained in a specialized tumbling media reactor. However, aluminum oxide does not clump and so can be held in a mesh bag and placed anywhere that it receives some water flow. Chemical filter pads offer even more control over phosphorus levels as they both remove particulate matter and absorb dissolved phosphate.
Phosphates in Your Reef
Dynamic nutrient-cycles are critical to the vitality of your captive ecosystem. In the usual aquarium, nutrients can build up rapidly. Keeping low fish-stocking densities, avoiding overfeeding and frequently replacing water changes is necessary to control nutrients such as phosphate. Because phosphate will (one way or another) find its way into your tank, you should check levels often. With some control over your phosphorus cycle, you can improve the health of your corals while limiting growth of unwanted algae!