Intuition is telling me your ppm's are too high. IF your ph and water temps are good, its ppm. Lower ppms will allow more 02 to be available to the roots, and i think its just generally burning the roots. Try keeping your ppm's at 150 to 250 until they get fairly large. dont lower the water too much. i used to lower the water but if you deal with the underlying issue you wont have to. Lowering the water means the plant has less water to buffer. This means more fluctuations in ppm, ph, and water temps, leading to decreased o2 availability. I use huge 30 gallon rubbermaids and the water level is up to an inch under my netpot, no problem. By the way for lightproofing your buckets nothing beats black 10mm construction plastic available at any home improvement store.