It's a little hard to figure out what that could be under that HPS light, but I am going to assume some nute water fell on that leaf and it got light burned. Don't see anything else, if you could take some other pics, maybe in natural light, but as far as I see, nothing there to worry.
Yeah it looks exactly like that to me as well. I often get careless trying to feed the plants and get under the foliage with the watering can. Spilled nutrient solution causes burns that look just like that.
Growing in dwc oxy pot bubblers 4 airlines to each pot
400 watt darkstar air cooled light with dual spec bulb
5 inch carbon filter
16 inch desktop fan
Air temp is between 24 and 27 deg c
Humidity between 60 and 70 %
Water temp between 20 and 22 degrees c
Ec at 1.8
Ph 5.8
Plants are 5 weeks old went into 12 12 yesterday give them a little longer as I was trying to do as much lst as possible !!
Using the shogun nutrient range !
Quick bit of info up until the fourth week I was running them at 1.5 ec I looked at my feed chart and realised it should be at 1.8 to 2.0 ec for there age so last week put the food up to 1.8 ec little bit of nute burn but nothing major ,
Where you mentioned it could be nute burn from spilling water onto leave but the only water that could of got into leaves is what had dropped from the humidifier where I have been filing it up and when putting it back to base some may have dropped on leaves but I have not done any foliage or watering the plants ! So do you think this could be where I have made the e.c solution to 1.8 , also can you notice any other problems you could point out ! Also the burn is only on a three fan leaves !
Thanks for your help also I'm struggling with uploading photo please could you go into my gallery to view Pics