Ahhhh the disappearing fish scenario.
I have two theories, the first is only applicable if you have several snails in the tank.
I've seen snails eating the corpse of dead fish before and I reckon a couple of snails will devour a dead fish in a matter of 36 hours or so.
The second theory is just that a healthy tank, with good denitrification, can decompose a corpse in a few days, quicker than the death is noticed ....
Batissa violacea? The seller didn't seem quite certain what they were. If they are they get quite big up to 15cm? They're already bigger than I was expecting. I assume they're alright it took 3 days for them to get here. But they were well packed in a poly box.
Now that they're in the tank they look white rather than blue which is a bit strange. Clams change colour from the stress of shipping? Should look good under evening/night time blue LED lighting.