Brown leaves

Hi, I am on my second grow and it is going well. The only problem is that one of the plants leaves are turning brown from the middle out. I read on a previous post that it might be a phosphorus deficiency so I added some more and it didn’t seem to help. I am growing DWC with Roberts Super Skunk and OG in separate 5 gallon tubs and all seem ok except for one of the skunks. The temp is 75, humid 50%, PPM is 635 and PH is 6.5. I am using GH 3 part series and just went to 12/12 light after 4 weeks of 20/4. Its odd that only one of the 3 skunks is doing this. The OG are doing good but I had to remove one because of a rotted stem. I am getting Cal/mag today, maybe that will help or are my nutrients just too low? I switched to a 700 watt LED light in a 4ft 3 by 3ft 4 x 7 room and they seem to thrive with it. I just have to watch the height off the plants as the plants drink a lot of nutrient water and the light puts out heat even though they tell you its not much. Any advise of info would be greatly appreciated. I also started 3 Gold Leafs in my grow box and they seem to be doing great after 6 days. Thanks 620smitty

I don’t think the PPM needs to go up, but the pH could go down quite a bit, in DWC it should be closer to 5.8, 6.0 would be fine but 6.5 is a little high to keep it at for any extended length of time.