looks like nutrient burnt to me, my plants got the same thing on 1 or 2 leafs but they can handle a good feeding, you might want to lower on the nutes or check out if there arent time releasing nutrients in it if you are feeding it straight water.
Also the leaves look a little hot to me, the bottom leaf is curling and they start to look a little like cotton or something like that, my plants only get that if i dont water enough if temps are high so you might want to water more or more frequently.
If none of these are the problem then they might need nutrients tho i doubt that since they wouldnt show the "burn" and only the yellowing.
Hope you find out quick cause they look good!
Peace!
