Somehow, even though we may not be aware of it, the phosphate levels dissolved in our aquarium water have a tendency to rise unless, of course, we do something about their presence. This is especially true when an aquarium has not yet aged for a long enough period of time. It also happens in aquariums that are not operated using the more recent reef aquarium principles.
By not using such principles I mean that the aquarium is either a fish-only tank with dead coral in it, or it is a reef tank that does not use the live sand and/or the liver rock method of filtration.
Phosphates are present in the aquarium in many forms. The one form we are interested in most is ortho-phosphate, which is the chemical form that algae can very easily use as a nutrient for growth, and will gladly do so in it's presence. Even though there may be complex forms of organic or non-organic natural compounds, they eventually all break down to ortho-phosphate when given enough time.
When the levels of dissolved PO4 rise above 0.03/4 ppm, the likelihood is high that micro-algae will start to appear in your aquarium. High is probably not the correct word to use as at such seemingly low levels micro-algae will appear for sure. This has been my experience and that of many others I know in the hobby here.
By micro-algae I mean any type of algae that we consider as undesirable in the aquarium, whether hairy, crusty, encrusting, filamentous, slimy and so on. Anything that we would consider a weed if we were talking about a lawn or garden applies here.
You may very well still believe that nitrates are the main cause of this algal growth because older literature has so often intimated that nitrates are the culprit. Although nitrates do contribute to algal growth, phosphates are a far greater reason for their appearance and proliferation of algae throughout your aquarium. Nitrate levels that are high are certainly not desirable but they appear to affect other matters in the tank more than actual algae growth (color and appetite seem to be affected in fish, and in corals not opening as wide appears to be a consequence of high nitrate levels).
You can test this is you wish by loading your tank, and overfeeding, so phosphate rises quickly, or you can take my word for it. Make the levels rise to 0.05 ppm and you will see that micro-algae of all kinds appear. I am not suggesting you should but if you are into "seeing is believing" then that would be the way to go about it.
As far as I was able to determine, Thiel was the first one to point out that phosphate was the problem and I believe his company was the first one to come out with a product, or compound I should say, to absorb phosphates. Who was first is not that crucial though, the key is that phosphates give rise to micro-algal growth. By now, I think, just about everyone is aware of that. I have found my personal email with him very informative and a good learning experinence but draw on knowledge from others as well and add to that what I discover myself. I certainly do not rely on just one source of information as that would lock me in to one person's theory and that would be sort of like creating tunnelvision after a while.
To keep our aquariums free of algae we can use several methods (I described them in another article I wrote). All those methods dealt with the outcome though (the algae) and not with the cause (the phosphate).
The article referred to, explained how to rid the aquarium of algae and how to lower phosphates and suggested a combination of methods to arrive at that result. Still the key is to understand that to really solve the problem one needs to eliminate the cause of the algae growth and not just the outcome of too high levels of PO4 (the algae).
The real problem we need to overcome however is the build up of phosphates in the aquarium water and we need to find the reasons where they come from. This is the aim of this article: to determine the causes of phosphate increases. I cannot offer solutions for each of the sources of phosphates as that would make this article much too long. If you look at all the causes though, you can start determining whether they apply to your aquarium and if you should do something about them.
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