You can switch them over VERY eaisily. I usually just take the bag that i get em in at the LFS, open it up, hang it over the side and just add a lil saltwater to it every 10 minutes or so. I've done this several times this way and like bang guy said, you can do it in an hour or so. I bought a pregnant black molly from the LFS a few weeks ago, changed her over to SW, then she had 5 BIG babies, these babies were HUGE. They've been doing really good so far and have grown alot.. If you put 5-7 mollies in a tnak, done be suprised when you have 50-70, lol... They're like rabbits. Good luck to ya!