Alittle more info, is the leaf soft or dry, better description of the color. i had a couple leaves turn a dark purple looked brown but the shit was soft as a baby bottom, so i knew it wasnt over fert, i think it was a K def. Also if its not really affecting the plant i wouldnt worry about it too much, if it worsens i would. Run some clean water pHed trhough the roots, what happend to me is my pump stopped working for a day and the plants only got waht little nutes were at the bottom of the rootcontainer and it locked out alot nutes i just washed the roots thuroughly and then went back to regular feeding/nutes. do u let ur tapwater sit over night to dechlorinate? chlorine will def. fuk ur sht up.