Phosphorus is a nutrient, and can be in either organic or inorganic form. Plants, algae and phytoplankton actually require Phosphorus for photosynthesis, and it’s essential to all living things, but if there is too much Phosphorus in the marine aquarium, known more commonly as Phosphate, it will cause nuisance algae.

Sources of phosphate
Phosphate is introduced into the aquarium in a number of ways. Tap water contains phosphate, which it gains from fertilizer run-off from farmer’s fields. That’s why reefkeepers use reverse osmosis (RO) or deionised water (DI), as it’s pretty much phosphate free. More about that in a minute.
Phosphate can also come from fish food, and many a reefkeeper has had a shock when phosphate testing the juice from their frozen food. Phosphate can be present in low grade salt or carbon, and even decor, but good carbon and marine salt will state that it is phosphate and nitrate free - and most do.
Calcium carbonate can give up phosphate, so when coral gravel is used in calcium reactors, the slight negative effect is that as it dissolves and releases buffers, it may also release some phosphate.
Dead fish, dead corals, inverts, macro algae or even bacteria can all give up phosphate as its in their very make-up when they’re alive. And uncured live rock will dump phosphate so should never be cured in the main display system.

How to control it
Luckily phosphate can be tackled in many ways, and due to the arsenal of products now available, can be eliminated almost completely.
Step 1 is to try not to add it in the first place, so no tap water should be used, even for set up, and ideally coral sand and ceramic rocks should be pre-soaked in RO water and tested before and after.
When the tank is newly set up, feed very sparingly, and rinse frozen food if using it. If anything dies, remove it quickly, and if any detritus is building up anywhere, syphon it out to remove it from the system.

Long term control
Still the most popular form of phosphate control is to use phosphate removal resins. Iron or aluminium based, you simply place them in a fine net bag anywhere in the system, ideally in an area of strong water flow. But the best upgrade for those resins is to place them in a fluidised reactor, which tumble the resin, ensuring that all of it is exposed to phosphate-laden water, and has maximum surface area exposed, with no channelling. Place the reactor into your sump or a rear filter section, connect to a powerhead, and change the media once a month, or when phosphate levels begin to rise. Throw away spent media.

Next is to tackle it nature’s way.
As we learned earlier, plants and algae use phosphate, so by purposefully growing macroalgae (seaweeds) or even mangroves, when also offered light, they will help consume phosphate and nitrate. Harvest some of the new macroalgae growth and you’ve locked up and removed some phosphate nutrients from your system.

Chaetomorpha algae, known as Spaghetti algae, or Chaeto, is compact, hardy, fast growing, and easy to harvest and remove. Caulerpa, a higher form of macroalgae, is a little more troublesome. Grow Chaeto in a specially lit area of your sump called an algae refugium, or turbo it by placing it in a specially designed algae reactor, where it gets a strong flow of nutrient laden water, and plenty of light. Algae scrubbers also exist, where simple algae is grown over a plate, exposed to water, air and light, although reactors and refugiums are much more popular.

Macro algaes will also consume ammonia and nitrite, act as a biological media, and provide an amazing refuge for tiny beneficial creatures like mysids and copepods. The slight downside is that being a plant, it will consume other elements from your reef tank water like Iron, competing with the photosynthetic corals for it, and some believe that some algaes will release allelopathic chemicals into the water to halt coral growth and compete for space.

Probiotics
Probiotics use another gift from nature, bacteria, which, when heterotrophic bacteria are given an additional carbon based food source, consume nitrate and phosphate just as quickly and easily as nitrifying bacteria consume ammonia and nitrite. Depending on the brand, you either add special strains of bacteria with one bottle and a food source with another, or just a food source, and bacteria develop and consume nutrients on their own. Probiotics don’t need macroalgae or light and just like with ammonia and nitrite, a bacterial population will exist to consume nitrate and phosphate as and when it becomes available. You do need to dose the food source regularly - daily in some cases, but you won’t get the performance drop-off that you’d get if your resin was spent and not removed and replaced. And you do need a protein skimmer, to skim off old bacteria and their waste.

Probiotics tend to lower nitrate quickly and easily, but phosphate removal can take longer. A nitrate level of zero but more of phosphate can affect something called the Redfield Ratio, which despite both levels being low, they are out of sync, and nuisance algae can still take advantage even then. This is when you hear of reefkeepers actually trying to raise nitrate and phosphate by increasing feeding, and backing off on the regular probiotic dosing.

Bio pellets
Pellets are highly effective at reducing nitrate and phosphate, which they do with carbon based polymer beads inside a reactor. Water is pumped in at a high enough rate to tumble the beads at speed, where nitrate and phosphate bacteria colonise, and feed on nutrients. Once mature a pellet reactor can control both algae nutrients, you don’t need many pellets per tank volume, and they last a long time. The reason more people aren’t still using bio pellets is that the job can now be done purely with probiotic bacteria, and anecdotally, nutrient levels could drop dangerously low, although this is also possible with probiotic bacteria. See below. A protein skimmer is also recommended when using biopellets, to remove old biofloc.

Mechanical removal
Sources of phosphate can also be removed by other, more simple means. Any dirt you syphon or net out will also contain some phosphate, as will removing uneaten food. A filter sock will capture physical waste and with it phosphate, but if you leave a dirty sock running in a sump, it will also leach phosphate into the system as that trapped waste breaks down.
Protein skimmers remove phosphate, as a test of skimmate will prove, and roller filters also physically remove dirt and some phosphate, before that roll is removed from the system and disposed of. RO and DI units physically and chemically remove phosphate from tap water, so regularly test the phosphate level in your source and product water, as you may be adding it by mistake in your top up water and water change water if your RO cartridges need changing.

What is the right phosphate level?
The phosphate level in the oceans is approximately 0.07mg/L. At this level nuisance algae is kept in check along with phytoplankton and macroalgae. It’s a biolimiter. For the home reef aquarium aim to get levels as low as 0.05, but many people have much higher phosphate levels, and there lies the problem. In high levels, phosphate can even retard the growth of some corals, although their symbiotic algae don’t mind it and flush brown when kept in elevated levels of nitrate and phosphate.

Go below 0.05, and you’re then into something called an Ultra Low Nutrient System, or ULNS. The advantage of running Ultra Low nutrients is that the brown pigment is then stripped back from sps corals, leaving light, bright colours like pinks, blues, yellows and greens. You will also have little to no nuisance algae growing in the system, even in brightly lit tanks. The disadvantage with ULNS is that corals have less energy from the zooxanthellae and need more feeding. They are also less resilient to water parameter changes in the tank, less hardy, and more likely to lose all their zooxanthellae, and bleach. Not everyone agrees with running nitrate and phosphate levels so low, and clams actually need phosphate in the water, as do many soft corals and lps corals. Go ultra low and your other corals may suffer.

Can you mix methods of phosphate removal?
Yes, you could certainly have mechanical filtration, a skimmer, and an algae reactor, you could also run some resin in a reactor, although then you will starve the good algae of some of its food. Pellets and probiotics are best run on their own with mechanical filters and skimmers, as you are trying to build and maintain a bacterial population, which itself would then be starved by physical adsorption by resins, or take up by macroalgae. Many reefkeepers run many different methods however, and most experienced reefkeepers have tried all of the above, and will swap and change as they see fit.

Testing
Phosphate should be tested for regularly, with an accurate, low range Phosphate test kit. Advanced testing by ICP-OES will also reveal very accurate analysis of Phosphorus, and tell you how much is too much. Silicates are also linked to nuisance algae growth, can be detected by ICP test, and can be removed by some phosphate removal resins at the same time.