molds and mildew spread by air.. ventilation is important .. never add water or spray plants when lights are off. remove excess water. if mine i'd seperate them..
if not sure about water, buy a moisture meter with probe that can reach down in the pot. for less than 10$ you can water then right till you get it figured out.. plants perk up when lights are turned on.
nitro deficient .. try foliar feeding with a spray bottle.. i use and like earth juice organics foliar feeding allows the plant to use it quicker. with fertilizer more is not good . err on the safe side using less than required at first .
i'd start by using a much smaller container to grow in. when you use a huge pot for a small plant the chance of over watering increases.. if new to this but an in expensive ($10) moisture meter. and check before you water them..More plants die from too much water than most other causes...
pot too large. i use 7 gallon pots and have nice big bountiful plants. 15 is way big unless you have unlimited space and time. keep in mind many strains don't grow too tall and will fill out .. the amt. of light is important. in ideal conditions they will get so big around you won't...