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Originally Posted by bjbauer
...does the rock and/or the crushed coral actually leach phosphate? Forget for the moment if detritus that can build up can release it. The amount of crushed coral is like 1/4 to 3/8 inch. If the rock or coral is leaching phosphate a test should indicate it. If it is highly likely however, I would just as soon remove it now rather than later...
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Calcium-based substrates, including rock and the aragonite or calcite sands,
adsorb the phosphate molecule to their surface. Bacteria in the substrate as biofilms, as well as nuisance and macro-algae can store the phosphates in varying degrees, and will all rapidly and fiercely compete for these relatively scare resources in nature (as well as in closed systems, but phosphate in closed systems is not a scare resource). The nature of coarse crushed coral substrates is such that they easily
trap detritus, left over food items, and miscellaneous organic substances that all rapidly decompose to their constituent compounds. It is this decomposition and release of these substances that is the bane of the
crushed coral substrate.
Such coral-sourced substrates from old systems do not have much ability to release these phosphates at ASW aquarium normal pH (8.0-8.4), but in the sandbeds where bacterial resirations can drop the pH due to the production of organic acids and CO2 release, de-adsorption can easly occur for phosphates and phosphate-containing organic substances (organophosphates), and bacterial production of phosphatases (phosphate releasing enzymes) CAN release many complexed organophosphates that would otherwise not be available to fuel nuisance algal blooms.
The presence of high levels of phosphate will not be detectable in hobbyist's-level test kits, due to the organic complex formation as well as potential formation of other undetectable forms of phosphate ion species. Test kits for the hobby detect only free phosphate ions/molecules, and in a biologically active system (all of our tanks), phosphate does not stay in this form for more than a very short time. Detection of phosphates is quite difficult for this reason, and testing will not reflect a true measure of what the actual amount of phosphate is in the water.
Phosphatase enzymes acting on organic compounds and new releases of phosphates from substrate due to pH shifts make biologically available forms of this nutrient for absorption by algae. They can quickly assimilate it from the water and trigger algal blooms. This is the issue with reusing old substrates, and although it may seem wasteful and expensive to toss the old substrates, replacing them before there are algae problems is MUCH easier than dealing with nuisance algal blooms. Rock can be "cooked" to purge them of excessive phsphate content. This is not boiling the live rock, which kills our desirable beneficial bacteria, but a biologically-driven purge of the rock of its excess phosphate content by allowing the bacteira in the rock to grow without light (and competition by algae). The bacterial colonial growths push the bacterial cell mass containing these phosphates out of the rock as detritus, which is then either skimmed from the water via protein fractionation or siphoned as surface detrital mulm from the surfaces of the rock and the bottom of the purge container. Do a search on the topic in the archives for more detail. The rock can in this manner be treated to release its excess phosphate content, unfortunately, the sand cannot be treated in this manner.
HTH explain the issue.
