Mine did the same. I bought 3 assassins and they wiped out the nuisance snails real quick. From what I read assassins are not asexual like other snails so just so happened I ended up with at least one male and one female out of the 3. The do breed really fast. I actually like them since the burrow in the sand and eat leftover food.
If I drop hikari pellets for the corys the assassins will come out of the woodworks and swarm the pellets