he has a 400w lamp about 20in away from 3 indica/sativa hybrids with desk fan and a carbon filter for air circulation in a 6ft tent. gives them about 300ml of water every other day. the soil has enough nutrients for the first few weeks which has not passed yet and the are in vegetative stage. the problem is that his bottom leaves were starting to go yellow at the tip but were cut off early but now the new leaves at the top have yellow tips on 2 of them.....any ideas please before it gets worse?????????????????