Assassin snails sudden death

[HypodeemicNerdle]

Anyone have experience with these snails?

I bought 7 of these to deal with an explosion of pest snails and have kept them in a separate tank in case they were carrying any diseases. I have been adding pest snails to the tank for them to eat which they have been happily doing for about 3 weeks.

Suddenly this morning 6 of them are dead, they are protruding from their shell and not moving.

The pest snails in the tank are happily moving around, the water tests fine, the only hint anything was wrong was one snail had a small piece of green plant growing on its shell, it didn't seem to bother it but I don't know what is was.

What happened to my snails? They weren't cheap and we're doing a good job

[EauRouge]

Are they definitely dead? They don't move around that much.

What's the pH in your tank?

[HypodeemicNerdle]

The pH is 7.5. 2 of the snails have upgraded themselves from dead to alive which I am very happy about, the other 4 still appear dead, but I'm not going to throw them away yet incase they are hibernating or something