Why Do My Assassin Snails Die?

[trianglekitty]

Any idea why my assassin snails keep dying?

Water parameters are normal, PH is 7.5, temp is 79 degrees, and the tank mates are not aggressive (cory cats, guppies, and rasaboras). I started with three, and in two months I'm down to one, which spends days at a time hiding inside a piece of driftwood (I've never seen it eat). There are plenty of pond snails, plus I feed bloodworms for the cory cats, so there is food for it.

I'd like to get more, but not until I figure out why I can't seem to keep them alive.

 

[Randomitity]

They might not be finding the other snails fast enough before they starve

 

[trianglekitty]

They might not be finding the other snails fast enough before they starve

I thought of that, because again, I've never seen any of them eat. But I did find empty shells when I cleaned the gravel. I've even tried putting the assassin snails in a bowl with snails from the tank, and they didn't eat any. I've seen them crawl right overtop of pond snails and keep on going. Right now there are plenty of snails everywhere to support a single assassin snail without much effort. So I don't think it is that food isn't there or they don't reach it, they just don't seem to be eating it for some reason?