NOt all snails are bad by anymeans. I wouldn't exactly say the stuff in the weekend feeders is plaster... but well. I actually feed them to my apple snails, they contain good calcium in that plaster stuff and really help their shells grow well. However, I would never use them for their intended purpose...and htey wouldn't find there way into any non-snail tanks. I do find that the phsya pest snails (the foot ball shaped ones) can be bad and harder to control. They are ones that will harm my apple snails. The planorbids (spirally ramshorn snails) aren't as bad, and can be used as population control for the phsya, and then controlled themselves by feeding less. The lettuce trick works well, but you've got to stay on top of it or you'll get another population explosion. As long as you keep the physa population in check though, it should be fine. They are a good clean-up crew, and these snails will actually eat algae...
Emily