I have the very same problem, but this is my first ever attempt, so I put it all down as a learning curve.

The first leaves (germination leaves) went so yellow and dry they have fallen off, the second set of leaves are yellow at the tips and appears this set of leaves will also die off, the rest are all fine and a nice shad of green, I was watering them every day but have stopped watering them, and now only water every 3-4 days to see how that does, also I had used Fertiliser in the form of baby bio 3 days in a row about a week ago, but in very low amounts, so have stopped that as well to see how it goes. As I say this batch is all a learning curve for me, and finding it a great joy constantly reading and finding out new things, even though the above sounds pretty bad most of them are now 1 & 1/2 foot tall, and already smell strong of loveliness

My biggest concern is I am using a 400w MH in an old converted all metal 6ft by 3 by 3 tool shed within a wooden shed, enough room for 5 pots, but the heat this light chucks out worries the hell out of me while im at work, in case it burns my shed down then my house which is right next to it...