looks like the mg soil is burning the plants. i've heard horrible things about it in the past although i've never used it myself. also you should wait till the pot is almost completely dry(not just the top) before watering and 1 plant per pot is going to be in your best interest as well. if you are going to upgrade to a hid you should conisider hps instead of mh, you will get much more/better buds flowering with hps. otherwise, keep up the good work and always listen to what your plants are saying, they will tell you what they want all you have to do is pay attention.