What MarAquatic said. Your PO4 levels aren't that far off from what reefs can be exposed to in nature.
I would encorage you to get Forest ROhwer's "Coral Reefs in the Microbial Seas" Current research is showing it's not niteates and phosphates that causes coral mortality but excess labile DOC, or Dissolved Combined Neutral Surgars (DCNS) that promotes pathogenic bacteria. Rohwer's book is an excellent and very readable introduction.
Unfortunately a couple decades ago the notion PO4 has to be kept at unnaturally low levels became a mantra.
Kleypas 1999 and
Conkright 2000 both showed higher PO4 levels naturally occurring than was being recommended at the time on the forums. J. E. N. Veron, one of the worlds formost authorities on corals reefs, made statement over 25 years ago ""Imported nutrients are usually transported to reefs from rivers; but if there are no rivers, as with reefs remote from land masses, nutrients can only come from surface ocean circulation. Often this supply is poor, and thus the vast ocean expanses have been refered to as "nutrient deserts". The Indo-Pacific has many huge atolls in these supposed deserts which testify to the resilience of reefs, but the corals themselves may lack the lush appearance of those of more fertile waters. Many reefs have another major supply of inorganic nutrients as, under certain conditions, surface currents moving against a reef face may cause deep ocean water to be drawn to the surface. This "upwelled" water is often rich in phosphorus and other essential chemicals."
Corals of Austrailia and the Indo-Pacific pg 30
More recently reseaarch done at Southampton University in England showed insuficiant PO4 made them pale and very sensitive to either changes in lighting or temperature. Here's links to their research:
High N
ratio, Nitrate enrichment can increase the susceptibility of reef corals to bleaching
http://www.indiaenvironmentportal.org.in/files/file/Nutrient enrichment.pdf
Ultrastructural Biomarkers in Symbiotic Algae Reflect the Availability of Dissolved Inorganic Nutrients and Particulate Food to the Reef Coral Holobiont
https://www.frontiersin.org/articles/10.3389/fmars.2015.00103/full
Phosphate deficiency promotes coral bleaching and is reflected by the ultrastructure of symbiotic dinoflagellates
https://www.sciencedirect.com/science/article/pii/S0025326X17301601?via=ihub
And here's a couple more links on PO4:
High phosphate uptake requirements of the scleractinian coral Stylophora pistillata
http://jeb.biologists.org/content/214/16/2749.full
Increased PO4 Increases Acropora murcata's growth rate.
https://www.sciencedirect.com/science/article/pii/S0022098111004588
Here's a couple links on Labial DOC:
Excess labile carbon promotes the expression of virulence factors in coral reef bacterioplankton 2017
https://www.nature.com/articles/ismej2017142
Coral and macroalgal exudates vary in neutral sugar composition and differentially enrich reef bacterioplankton lineages.
https://www.ncbi.nlm.nih.gov/pubmed/23303369
Sugar enrichment provides evidence for a role of nitrogen fixation in coral bleaching
https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.13695
