Just FYI but phosphate standards are avaialable online if you want to verify your test kits. Phosphorus cycle in reef systems is very complex. Keep in mind we can't test for particulate or dissolved organic phosphorus. And besides corals and algae messing with it sponges are messing with it and biofilms can sequester quite a bit. FWIW .2 mg/l is what corals are exposed to on many reefs in nature and some systems can run much higher. I'd also point out work done with corals in an aquarium at SOuthampton University* in Engalnd identified a threshold level of .03 mg/l to minimize the risk of phosphate deficiency in corals
Fig. 4 from this
paper
Here's a link to an article and video by Rich Ross on phosphate
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Richard Ross What's up with phosphate
Links to Southampton's research:
An experimental mesocosm for long-term studies of reef corals - Volume 92 Issue 4
www.cambridge.org
An Experimental Mesocosm for Longterm Studies of Reef Corals
Phosphate Deficiency:
Nutrient enrichment can increase the susceptibility of reef corals to bleaching:
Increased dissolved inorganic nitrogen (DIN) concentrations in sea water have been linked to a reduction of the temperature threshold at which corals bleach, however, the mechanism underlying this change is not known. This phenomenon is now explained in terms of increased phosphatase activities...
www.nature.com
Ultrastructural Biomarkers in Symbiotic Algae Reflect the Availability of Dissolved Inorganic Nutrients and Particulate Food to the Reef Coral Holobiont:
Reef building corals associated with symbiotic algae (zooxanthellae) can access environmental nutrients from different sources, most significantly via the up...
www.frontiersin.org
Phosphate deficiency promotes coral bleaching and is reflected by the ultrastructure of symbiotic dinoflagellates
Enrichment of reef environments with dissolved inorganic nutrients is considered a major threat to the survival of corals living in symbiosis with din…
www.sciencedirect.com