in a lot of the stores around here, they use display tanks are in units like the picture shown below. these kind all share the same water through a sump system in the bottom. either way, there is always a chance at introducing somthing unwanted into your tank. more than likely, they use the same nets and stuff to catch them anyway. i would get them at about two inches or so long. some simple comet goldfish will work just fine if that is the rout you would like to go. just take a good look at them before you buy. if they look sick, get somthing else.