When to move out of hydrodome??

Hey guys I just killed a bunch of seedling. I moved them from the hydrodome to the flood table. They are in rockwool and were moved to hydroton. The roots were starting to poke throught the bottom of the cube. They were started 9 days ago. I think that it was too early. Some are still looking good but most died. I was just wondering if there is some sort of standard timeline to follow from seedlings. I mean an easy to follow chart somewhere. Thanks. I will take any advice.