Phosphorus and your Aquarium

Aquatic animals and plants held in ponds or aquaria will have a significant effect on the chemical composition of the water. One of the major trends that can be expected to occur in aquarium or pond water is the accumulation of waste-derived chemicals. These may be in the form of toxic compounds such as ammonia which must be converted into a non-toxic form or non-toxic compounds such as phosphate which can still cause other problems by encouraging the growth of unwanted algae.

Phosphorus is a vital biochemical. It is part of bones (calcium phosphate), it is found in the subunit molecules that make up DNA and is found in numerous other vital biochemicals. Plants commonly contain 0.05 – 0.5% of their dry weight as phosphorus and fish usually contain a figure of 2 to 3% phosphorus or more. Phosphorus usually occurs in animals and plants as phosphate (PO43-).

Sources of phosphorus in ponds and aquaria:

One of the main inputs of phosphorus in aquaria and ponds is fish food. Artificial diets like flake and pellet foods have phosphorus concentration in the region of 1%, so after each feeding session the amount of phosphorus cycling in the aquarium or pond will increase. Animals and plants will both excrete phosphate into the water as they metabolise. Another significant source of phosphorus into the water is the rupture of dead cells. Fish will be constantly sloughing dead cells into the water, plants will release dead tissues into the water, where the cells will again rupture and release their contents.

In the natural environment sources of phosphorus are primarily weathering of rocks and break down of sediments. This releases phosphorus into the water and accounts for most of the global phosphate. Another significant source however is agricultural fertilisers, animal manures and municipal sewage which both find their way into river systems and thus eventually to the sea where it cycles between surface water and the sea bed sediments.

Phosphorus  in water

Phosphorus will cycle through numerous forms in the water. Dissolved Inorganic Phosphorus (DIP) is the most abundant form and is also known as Orthophosphate, reactive phosphate and phosphate. Here the phosphorus is dissolved in the water body usually as PO43-,H2PO4 or HPO42+. The acidity of the water determines the proportion of these phosphorus species in the water.

Another form of phosphorus in water is Dissolved Organic Phosphorous (DOP), and this accounts for the phosphorus bound to organic compounds dissolved in the water. Finally a significant proportion of the phosphorus will be bound to particulate matter in the detritus and sediment, and this is known as Particulate Organic Phosphorus (POP).

Problems with phosphorus

None of the forms of DIP, DOP or POP are directly toxic to fish at normal conditions so phosphate is not a direct fish health concern. The problem with elevated levels of phosphate in aquarium or pond water is that it encourages algal growth. Phosphorus is required for plant growth, and excessive levels of phosphate can mean excessive growth of plant life.

In lakes and rivers the enrichment of the water with nutrients is termed ‘eutrophication’ and this can lead to heavy growth of algae or aquatic plants. These nutrients are mainly nitrate and phosphate, with phosphate being the limiting factor in freshwater environments. In other words it is the presence of phosphate that triggers the plant growth, and it is phosphate that most commonly runs out first.

Because phosphate is a limiting nutrient in freshwater environments, freshwater algae and plants (macrophytes) have evolved to be able to store phosphate inside their cells as Condensed Inorganic Polyphosphates. This enables the plant or alga to use the stored phosphate when levels of DIP in the water have dropped to growth limiting levels. This phosphate storage is known as ‘luxury consumption’ and may help to explain why controlling algae in ponds or an aquarium is so considerably difficult.

In marine environments phosphorus levels tend to be more stable. The level in natural seawater is around 0.07 mg/L, with higher levels recorded inshore due to pollution and river inputs. Consequently nitrogen tends to be the limiting nutrient in marine environments and marine algae are not quite so thrifty with their phosphorus. Marine seaweeds are known to excrete DIP and DOP into the water.

The level of Phosphate in marine aquaria is generally far higher than natural seawater so marine algae such as Caulerpa may be adding to the overall phosphorus level as well as lowering it.

Green seaweed (Ulva compressa)

Control of phosphorus – getting rid & preventing accumulation

The inputs of phosphorus in a pond or aquarium are primarily fish food, fish and plants. So surely the best way to keep phosphate levels down is to reduce the amount of fish plants and food, but where is the fun in that!

The key to controlling phosphorus is to increase the outputs of phosphorus in the system so they are more balanced to the inputs. The most obvious way to lower phosphorus levels is to change a portion of the aquarium or pond water for fresh treated water that is free of phosphorus.

Freshwater aquaria and ponds where higher plant growth is encouraged should show a reduction in levels of DIP in the water as the plants assimilate and grow. If the phosphorus demands of the higher plants leave a surplus DIP then algal growth can be expected. Another way phosphate can leave the water is through aeration. When water is aerated orthophosphate becomes bound to dissolved organic compounds (DOC), in the water. This DOC is attracted to the surface of air bubbles, (A process exploited by protein skimmers or foam fractionators), so as air bubbles rise to the surface and burst the orthophosphate leaves the water via minute airborne droplets of water known as an aerosol. One study showed how 90 % of the orthophosphate was removed from a sample of seawater after 24 hours of aeration with an aquarium air pump.

Using aeration or protein skimming in a marine aquarium is thus a useful and simple tool in phosphorus management. It is unlikely that this technique would work as efficiently in freshwater as it forms and hold air bubbles less readily than seawater.

Liquid additives can also be very effective at removal of phosphates from aquarium water, Tetra PhosphateMinus can remove 2 mg/l of phosphate when used as instructed and will cause no water clouding, KH reduction or pose any threat to aquarium inhabitants. Phosphate removing chemical filter media are also very useful in removing DIP from the aquarium or pond. These rely on a natural attraction for negatively charged phosphate (PO43-), to positively charged molecules such as aluminium or iron.

Phosphorus becomes bound to organic material as we have seen above. Also a portion of the total phosphorus in a system will be Particulate Organic Phosphorus (POP), which will collect in the mulm or bottom detritus. The regular substrate cleaning in an aquarium or pond an efficient mechanical filtration will also help to manage total phosphorus levels in the water.

Phosphate accumulation in fishkeeping systems is inevitable and significantly increases the risk of troublesome algae growth. It should be tackled on numerous fronts, but limiting inputs through sensible feeding regimes of quality fish foods. Phosphate removers are effective also, and higher plant or macroalgae growth will also help lock up phosphate out of reach of the algae.