Maybe Tilapia...but then that becomes a question of how big is your "tank".
I always add in those mystery snails from Petsmart since they do not lay eggs within the water.
I know you said they were mystery snails, but do you have any idea what kind or any pictures to help identify? This may be a solution for a friend that wants snails but want easy egg cleanup.
Looking for an excuse to get back into fish keeping. Hopefully this will be it!