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Salifert's kit and most other kits only measure inorganic phosphate. The Merck/Deltec kit and colorimeters measure orthophosphate which is what gets metabolized.
By the time you see HA growing it's way too high. I judge by how long it takes for a diatom film to grow. Any less than 4 days and nutrients have built up. On the Merck kit I get .008. That is plenty for the corals to grow. Anything over .02 on that kit and I'm just feeding algae. Again that is orthophoshate levels not inorganic levels.
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