looks to me like the beginning of one of 3 things...magnesuim, phosphorus, or potassium...im leaning towards phosphures, only because of the dying spots right in the middle of the leaf...a pic of the whole plant would help...but i;d flush with some bloom nutes...lightly, and some cal/mag...or just flush with bloom nutes, and foliar spray some cal.mag...after lights are off of course. if you dont have cal/mag...then get some unsulphured molasses and add to your flush solution...lighly as well...molasses has calcium, magnesium...and iron in it.