iron..maybe

try some ironite, 1-0-1 wont burn, its mostly a pigmentation thingi

otherwise they look great. Nute burn normally attacks the tips first (they will start to turn brown & get brittle)
also every other day for feeding nutes is too much. it takes about 3-4 days for nutes to get consumed & washed thru the cycle

also misting, can cause this. water on leaf's + light. Creates a microscope effect and burns your plant