.5 mg/l PO4 is fine. Here's a good video to start with:
And here's a list of papers to read. The first three were done with a closed system at Southampton Univeristy started over ten years ago, so longer term than is often seen. And keep in mind corals are looking at the TOTAL amount of nitrogen and phosphate available, organic, inorganic, dissolved and particulate. PO4 and Nitrate are small parts of the overall nitrogen and phosphorus picture the coral holobiont is working with.
Phosphate deficiency promotes coral bleaching and is reflected by the ultrastructure of symbiotic dinoflagellates
https://www.sciencedirect.com/science/article/pii/S0025326X17301601?via=ihub
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
High phosphate uptake requirements of the scleractinian coral Stylophora pistillata
http://jeb.biologists.org/content/214/16/2749.full
Increased phosphate increases growth
https://www.sciencedirect.com/science/article/pii/S0022098111004588
Here's two quotes from two preeminate coral reef researchers:
"
When I see the colors of some of these low nutrient tanks, I can't help but be reminded of bleached coral reefs. It should therefore not come as a surprise that feeding corals in such systems becomes a very important component in these systems. Though reefs are often catagorized as nutrient "deserts" the influx of nutrients in the form of particulates and plankton is quite high when the total volume of water passing over a reef is taken into consideration.
Our crystal-clear aquaria do not come close to the nutrient loads that swirl around natural reefs. And so when we create low-nutrient water conditions, we still have to deal with the rest of a much more complex puzzle. Much like those who run their aquarium water temperature close to the thermal maximums of corals walk a narrow tight rope, I can't help but think that low-nutrient aquariums may be headed down a similar path." Charles Delbeck, Coral Nov/Dec 2010, pg 127
"
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." J. E. N. Veron "Corals of Austrailia and the Indo-Pacific" pg 30
Conkrite, et al, 1999 gives much higher phosphate levels for the ocean surface than the .003 mg/l sometimes recommended
https://gmao.gsfc.nasa.gov/research/oceanbiology/reprints/conkrightetal_DSR1999.pdf
