It natural that your plants are dropping lower leaves. It's robbing Peter to pay Paul. It wants to developt those leaves close to the lights. You can fix the leaf drop by adding a bit of grow nutes back into your mix. Say 1/4 dose to start and see if the leaf drop slows or stops. To me bottom leaf death is not that big a deal. It not like the lower growth was going to make anything all that special but sometimes I just like to see a healthier plant. At least until I'm 2 or 3 weeks out from harvest.