Check out the main site's
section on water quality. It has all the details on everything detailing with water and turtles.
Most people that have been on this forum for a while will also suggest against listening to most things a pet store says about turtles, since most pet store staff aren't that knowledgeable about them. What the pet store says might be good enough to keep them alive and they are probably being honest and trying to be helpful, but the main site here was made by people who try to do the best for their turtles, and have had tons of specialized experience with turtles.
The main thing you should do is start a fund for your turtles, because they're going to cost you a bundle. But you'll find out that they're worth it.
Good luck, and welcome to the forums!