it may be too much magnesium, i dont think the nitrogen is playing a role, and 12 inches away from the plants is way too far, especially with 400 watts. My advice is back off the nutrients, and flush it for a couple of days till it gets better. Also, your plants may be too small for that 5 gal bucket, so make sure to give them little amounts. what seed co. did you get your white rhino from, if the leaves start getting yellow and crusty, give the plants a straight shot of pure N.