Everyone died

[Hans-Werner]

The OP had an anthias that died a day before everything else. To me, that rules out a transient low oxygen condition, plus the clownfish surviving until the next day, but then dying also rules that out.

But it doesn't rule out some kind of intoxication, possibly with an organic/biogenic toxin.

However, my main point was that the snails and starfish and the shop selling them are not to blame for Amyloodinium deaths on Monday night. For this, the lifecycle of Amyloodinium and other parasites is too long and complicated.