Leaves at bottom of plant turning yellow

So I have now been growing for 2 weeks, 4 days. I used regular indoor potting compost and have been using a 250w metal halide for 18 hours a day since the start. I have used miracle grow pour and feed but have now purchased bio bizz. Had no problems whatsoever until around 7 days ago when one of the leaves started going a funny colour, the next day a few more leaves on some of the other plants were the same. Unfortunately i had to go away for 5 days so left them with water and on return they look similar, very healthy at the top but the bottom leaves starting to die at different rates. I'm guessing its too much nutes or too little so would be grateful for any advice. I feel bio bizz are better nutes but don't want to give them yet if they is already too many.

thanks in advance

thats def. burn from nutrients bro. just add water nect time it gets dry, and let it dry out. then 2 waters from now add half the nutrients u used that time u burnt them

thanks ill leave it for a while