usually flourecents set right above the plants will provide enough light till 3 weeks of age then you should move them to under a hid for best results and after giving them a few days to adjust even the hid should be kept as close as possible without burning the tops of the plants for a 400 would expect it to be7- 12 inches or closer with an air cooled hood.
its common to change out the solution on a regular basis suggest you follow the hydropinic system recomendation for a starting point it may be every week or slightly longer like every 10 days. you can top it off with regular ph adjusted water in between nute changes.