Is this nutrient burn ? Only on 1 of 3 plants.

Hello My friend I'm a dwc grower so let me have a little look into your situation here :) So your EC does seem a little high for this stage check out my Black sugar she is near enough the same age and My ec is set to 0.5 (disregarding the tap water ec) I have no problems but yes I would say its a tad bit too much for her at this stage lucky for you dwc is the best system in my opinion cause you can bin that water straight away and create a new one which should clear this up in about 2 days I would advise popping the ec down abit and then if it doesent clear up just use ph balanced water to feed until back to healthy though nutrient burn will often occur from the leaf edges and inwards so if you notice the sides of the edges getting brown too then thats defenatly an over feed. then jsut flush and you will be fine :) Hope this helps bro