Fish dead.

[Spyd]

Another big one to check is Alkalinity. If there was a larger ALK swing, it can affect everything in the tank as well.

Also, get an RO/DI unit when you can. It will save you many, many, many headaches. .25 for PO4 will cause nothing but problems with coral growth and algae. Best to run GFO in a reactor to help reduce that dramatically.