What kind of shrimp? I have assassin snails (a ton of them) in with my cherry shrimp and I've never seen them eat a shrimp. I have one lonely assassin with my yellow neos and CRS haven't seen min eat one either. I'm sure they have if given the chance.
A single assassin will do wonders in controlling a snail population in a 10g and under tank and is unlikely to do any harm to your shrimp so long as it's able to find it's preferred food, and with just one you won't have to worry about it reproducing and the population becoming a potential threat as food gets scarce.
Then again, I've got cherries, who breed like cockroaches. Even if the snail did take an occasional sub-adult, I'd never notice. Never had one take any of my adult females, though--and those I can and do take a nose count on regularly.
Never had problem with 5 assassin snails in a tank with 50 or so CRS. Ironically two of the assassins died because lack of food (my shrimp tank was free of pond snails). Or maybe assassin snail kill each other when there are no other snails to hunt?
I'd venture to guess that most likely if an assassin snail gets a hold of a shrimp, it's probably a sick one that is one the way out anyways. I just can't imagine something as slow as a snail being able to catch a shrimp. Maybe the snail has a stinger or something that can stun a shrimp...I don't know. This is just my guess. That said, I keep a single assassin in each of my 20g tanks and never had I had an issue. If by chance the snail does get the occasion shrimp, so be it. Granted I just have neos.
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i have 4 assassin snails in my tank, and the pond snail population doesn't seem to be crashing, lots of small ones, still a few large ones, still crushing 3-4 an evening myself.
never seen them catch a shrimp, and i've had fry since.
infact they have trouble keeping up with the pondy's from what i see, they just race on by.
have both in a 75 and in a 10 and i have no issues . Snails = slow shrimp = fast . Seems logical to me. I would guess that if an assassin gets a shrimp it is already dead or going to die. My shrimp clean his their shells off so they show no fear towards a potential predator