You have to put up more info to get a better answer. what nutrients, how much, what ppm or ec, what pH, how often do you change it? room temp, light type and height from plant? what do the roots look like? any slime buildup on them? does the water stink? are you using calmag? what's in the nutrients you're using? anything else you can think of? take pics of dry leaf in white light as close as you can get a good focus on those spots. Is this spreading to other leaves, if so, what do they look like, give pics.