Exactly what he said, although i feel for the first 1-2 weeks 24/0 is fine. I've just put my seedlings on 18/6 and it is starting to grow quiker again becayse after 1/2 weeks they slow down. They DEFFINATLY benefit from that dark period, because thats how it would be if they were groiwng outside, and basically thats wot your trying to simulate inside 18/6 all the way!