This is a tuff one....brown/rust spots...its like a mystery... some will say mag def....some will say pests...calcium def....nutes burn is even a possibility....I've went thru this before and never.found a clear answer...after hours of research...good luck
need more information, but the first thing to check is usually ph. make sure your ph is in the right range (5.5 hydro 6.5 soil). pictures and more info about your setup would help a lot with diagnosing your plants specific problem