[Van down by the river]

I'm going to go with poisoning....not from the Cuke.(obviously as he's alive)
When a cucumber goes, I agree it's all or nothing. The anemone appears fine, if it was decaying that's another story but it's not.

Could it have stung them? maybe, but many inverts are quite tolerant of anemone stings so I would suspect your other fish would go first. Mind you this is a possibility as firefish are scaleless, and Chromis can be delicate. But then your sailfin blenny should have been affected too.

When you were vacuuming did you pull up any smelly (blackish)spots aka: sulphur dioxide? It is a small possibility that you released this into the tank which would have caused the heavy breathing ( but so would released nematocysts).Your circulation would have dissipated it fairly quickly and the larger hardier fish may have been able to tolerate the diluted amounts.

I think that the dramatic changing of sand and 30 gallon water change are most likely the causing culprit. I don't know the full size of your tank but 30gal water change of new water and the stirring of the sand bed could have caused low oxygen levels as well.
I think you will never really know which it was or maybe a combination of some or all of these possibilities.