it's both... sometimes it can work but most of the times it doesn't.
what you want to do is.. an iodine dip (lugol's dip ) dood.
you dip your zoos and all the zoos you buy so you kill any bad hitchickers like nudis, snails (not sure how much that affect the snails), flatworms, fungus infection etc .
just make sure that if you are gonna do this you have everything good and ready dood
Didn't read that specific link yet but most people are using a freshwater/iodine(lugols)/furan2 dip. The Furan2 is there to kill the dreaded zoapox.
Check zoaid.com for specific instructions.
J