Note the Pocillopora in the middle of the photo.
A couple of weeks ago I was checking my sp. grav. against a new hydrometer. It showed the water at nearly 1.040 with the new one when the old on read 1.025. My conclusion: old hydrometers need to be replaced every couple of years. In the meantime rinse them in fresh water and occasionally soak them in white vinegar.

Before this I could not explain the bleaching of this and loss of some other corals. All other parameters were WNL.