+1 , never had puffers, I have had a tank that I could wipe 2-300 snails off the glass every week and never made a dent. 5 CL and 3 weeks of vacuuming empty snail shells out and then a thorough cleaning of my filter and haven't had a problem since. Put the clowns in at 3" and very quickly had some fat 4-5" clowns.
Only other way is to completely strip the tank let it dry for a bit and start over.
May want to watch overfeeding, I had to learn this myself so I'm not attacking you or anything, snail populations grow to the amount of food they have and they only get what's left from the fish. If you have a real bad case then you are most likely severely overfeeding. Again I'm attacking just sharing my experiences