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Sources of Phosphates

There are many reasons for phosphates to appear and then be present in the the water, and causes can be both external and internal.

Some Internal Reasons

• high fish and/or coral population in the aquarium,
• plant material that dies off and mineralization,
• breakdown of undigested food in feces,
• bacterial die-off and its decomposition and mineralization,
• algal die-off and its decomposition and mineralization,
• any other living material that dies, decomposes,
• fish slime (and coral slime in reef tanks) that decomposes (cfr Thiel)

Some External Reasons

• Overfeeding and the decay of the food that is not consumed,
• Phosphate in the water you are using,
• Phosphate present in the salt you use to make up water,
• Additives that contain phosphate,
• Carbon that leaches phosphate into the water,
• Calcium hydroxide of low quality used to make limewater,
• Phosphate-based chelators in additives (cfr Thiel)
• Phosphates in buffering compounds,
• Using frozen foods and adding the liquid after it thaws to the tank (cfr Thiel),
• Using low grade calcium carbonate in calcium reactors (cfr Knop)
• and many other

Observations and Comments

It should be pretty obvious from the above that there are a multitude of reasons why phosphate appears and rises in concentration in the tank. I have listed some obvious ones but the list is by no means complete.

It would be too lengthy to give you a plan of action that you can take for each of these cases. Suffice it to say that all of these possible causes need to be carefully looked at in your situation. Testing may have to be done to determine whether an additive is phosphate-free for instance. You may need to test your raw water and the salt you are using, and so on. Test your tap water, and test the water coming out of RO or DI units to make sure that the PO4 levels are near zero (you will need a test that measures in the 0.0x range to determine that).

Being aware of the possible causes is half the battle won! Taking action and eliminating the phosphate producing ones will allow you to get a handle on the problem. Doing so regularly will eliminate unwanted algae from your aquarium. Keeping some hermit crabs in the tank will help even more, but in my experience you do not want to have so many that they are starved for food and may become more aggressive, as explained in my other article.

Awareness of the fact that very low levels of phosphate lead to micro-algal growth is a definite step in the right direction. Many hobbyists used to think that several parts per million was a low concentration. It turns out that Thiel, and others after him, have proven that wrong. Levels lower than 0.1 ppm are already too high. Ideally you wish to maintain your phosphate concentration at 0.03 to 0.04 ppm.

As a last note: test your phosphate levels on a regular basis as it does not take much for their levels to suddenly rise beyond the safe zone. The death of one small animal in the tank, or overfeeding a few times can easily lead to levels that are higher than the suggested levels. Also, in reef tanks, additional die-off on some of the rock can increase the PO4 level. The more often you check your PO4 level, the better of you will be as you will have the chance both to intervene and lower the levels before algae starts to appear all over the tank.

Other types of algae

The algae dealt with in this article are for the majority green in color. If you run into red slime algae, you have a different problem. Cyanobacteria are the cause. I noticed that the Thiel web site has various articles already on how to deal with that situation. The same applies to silicates which lead to the appearance of brown diatoms. You may wish to read those articles to get a more complete picture of the algae situation in aquariums.

Whereas macro-algae may be considered desirable by some, this is a totally different topic. Of course macro-algae will feed on phosphates too. The low levels recommended and a small iron supplementation will make them grow and spread if that is what you wish to achieve. Remember though that many corals react negatively to the close presence of some types of macro-algae (Caulerpa types in particular).

From what I know, Thiel has done a lot of research on macro-algae so if you have any questions regarding them you may wish to join his mailing lists on his web site http://www.athiel.com and ask him questions. When I last visited him in Connecticut and that is a few years ago he already had close to 100 books on algae and phycology alone, not to speak of the hundreds of others on any subject you can think of that is closely or remotely related to the hobby.

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