I take them out of their pots. Wash away all the soil so you are down to the roots and then do a quick rinse with a bleach-water mix just to kill any nasties. I then rinse them with fresh water again and plant them in a clean pot with the soil that will be going into the tank. I then leave it on the window sill for awhile to make sure I haven't stressed it out too much and it's still growing. Then by the time my viv is ready to go, my back ground is all in and everything is the way I want it the plant is ready to go.