bleaching?

[Landolakes]

All of my euphelias and 1 bta have bleached. I made no changes except for adding more flow and new sand. All my softies are fine and my lta looks great.

ph8.2
no nitrates, ammonia or nitrite
checking calcium now but just did a really big water when I added the salt so that should be good, 100 gal out of a 180

 

[Landolakes]

OK calcium is low at 400, ph at 8.3 and my alk test only says normal no numbers. Please help as other corals and my lta have begun to bleach also.