Part of the confusion comes from a paper referenced by Randy Holmes-Farley that found very low levels of bisphosphonate inhibited calcification. More
recent research found "
Bisphosphonate has been considered to inhibit calcification but not photosynthesis in corals. We show that bisphosphonate may not inhibit formation of amorphous calcium carbonate and that the inhibition of calcification is possibly illusory" Research that's been around for a long time but overlooked or ignored by many is PO4 levels on reefs on average are .13 mg/l and upwelling can raise PO4 levels to as high as 2.0 mg/l. Here's a good video discussing PO4 levels in aquariums by Richard Ross from the Stienhart Aquarium in CA:
https://www.youtube.com/watch?v=ZRIKW-9d2xI
Phosphate is critical for corals if they are not being fed and their ability to use nitrogen is limited by it's availability. And research by
Southampton University in England found PO4 deficiency reduced coral biomass and promoted bleaching.