These situations can be incredibly frustrating. I think you almost need to make a list of possible problems then double, triple check them.
In terms of what hasn't been mentioned. Next time you get coral from a good looking tank test the salinity of the water that comes with it. Even though I doubt the issue is a bad batch of calibration fluid, it would be nice to check that off of the list of possibilities.
I think as others have stated an ICP test would be nice to rule out other possible problems and certainly bacterial biodiversity could be a problem. You're feeding regularly but I know some tanks for whatever reason have a problem with lower PO4.
In terms of what hasn't been mentioned. Next time you get coral from a good looking tank test the salinity of the water that comes with it. Even though I doubt the issue is a bad batch of calibration fluid, it would be nice to check that off of the list of possibilities.
I think as others have stated an ICP test would be nice to rule out other possible problems and certainly bacterial biodiversity could be a problem. You're feeding regularly but I know some tanks for whatever reason have a problem with lower PO4.