Wait untill your plants have 4 or 5 true setts of leaves. Then go to 12/12 on your lights. Wait untill you see nuts or white hairs coming out of the croch in the branches. You may already know that. Take out the ones that are males with nuts. Cut as much of the root off as you can. When you see the white hairs on females go back to 16/8 or 24 hours of light. this will put them back to vegg. Then you can take what females you want for mothers and cut the roots so they hang in the water an inch or two. This will only set them back a day or two, it doesn't hurt to cut roots back as long as they can get to water. I have done this before. The rest of the females if any you can put back to 12/12 to flower with a rez change and a good dose of hydrogen peroxixde to help with dead root in the water. You won't have many dead ones so it won't hurt that bad. Now you have mothers for cutting that can last two years or more.

Hope this helps and let us know how it's going. This help others to learn also.
Peace man.