Not a problem. If it is an asian freshwater leech you might want to keep an eye on those guys though, sounds like they could be a bit troublesome in the shrimp tank you mentioned, or in a tank with snails for that matter. If they are only going after dead stuff then they might be more helpful than harmful, could be you have a nice balance going on and they can get by without causing trouble, or I could always be wrong. Either way, if they've been around for a year and you havent noticed a massacre, sounds like everything's fine, whatever the case. ¯\_(ツ)_/¯