Wouldn't a sunfish be just as effective at snail control? Say spotted sunfish for example?
Absolutely! When I had bluegill or even yellow perch in this same tank they cleaned the snails right up. Not one visible. However the crappie have no interest in the snails.
I also got a couple hundred blue tilapia shipped Fed Ex overnight once, and put them in a tank with lots of tiny snails clinging to the circular tank wall. Within 20 minutes the tilapia (only about an inch) were going to town munching them down. It was like a feeding frenzy.
I normally have plenty of bluegill, tilapia, and yellow perch, but just not this time of year. They are in the ponds under ice right now.