Looks like not enough moisture in those leaves.... could be from lights too close, too much nutes in medium, too dry, too hot, too much wind, letting medium get too dry( one good dry out can cause lasting damage) or a strong foliar spray ( or sprays accumulating over time) whatever it is or was for that matter those leaves need more water in them not likely more nutes. Flushing them thru was good but I don't think the strong feed was the best option .... AN is fine just don't trust the "ph perfect" part it only really works for RO(20ppm or under) get a decent ph meter keep it calibrated and some strips to help verify keep it in the 5.8-6.1 range. Move your light up for few days see if it helps and if you have any fans trained right on them angle them up a bit or turn down to lower setting.... Make sure your water temps are decent too I like 70-75 but i use it straight away if it's gonna sit and warm before use( multiple waterings) slightly cooler might be better.... Hope it works out for ya either way;)
