cool setup, It appears from your pictures that the burn you are seeing is from nutrients, ease up on the plant food. every 4 days is a bit much IMO.
also if you setup your fan to blow right over the canopy aimed 1/2 way between your lights it will help to disburse the heat before it reaches your canopy and break it up a bit.
some times I will cut the tips off that start to burn, this hasn't done any harm. or just leave them.