One thing to note.. how close are the lights?
My last lot started canoeing slightly with raised edges.. it was high PH as the major cause, however even a couple weeks after that was sorted, I realised I also had my lights a bit close.
Looks more like heat than nutes. wouldn't the entire tip be going brown instead of all around the edges? (see pic, took that just now, lights/temps are sweet)
From what I've read, and I'm only 2 grows deep, they were trying to get rid of the heat.
Maybe something to look at.
If thats not the case.. at least I tried :thumbsup:

Good luck :rastasmoke: