Sherlock Holmes anyone ?

[Aquattro]

Did they start acting funny as you put the sand back? Maybe something in the bucket you were cleaning the sand in? To wipe out that much that quickly, it has to be something really nasty. I don't think algae going sexual would do that, and you'd see it in the water.
Siphoning too deep wouldn't do it. I would imagine that if the cuke "exploded", it would take out all or nothing.
LTA, not likely, same with the foxface.