Slightly yellowish leaves

I just now joined this forum. I have been growing indoors and out for about 20 years and have never encountered a problem that I couldn't figure out for myself until now. I started several Purple Kush seeds that I got from seedsman on (wait for it) April 20th. All the plants look sorta kinda good. They are now about 24" tall and in very large plastic pots on my deck outside. The problem is that three of the plants have yellowish leaves. Overall they look fine but many of the leaves at the top are a pale, yellow-green. I looked at the Ph of the water coming out of my tap and found it to be about 4.5 to 5.5. I live in Salem. I mixed up about seven gallons of water and Ph Up and got the tap water to a Ph of 6.5. I tested the city tap water before I added the Ph Up. I watered two days ago and have seen no change in color.

A bit of background. The seeds I planted did not get off to a good start. I planted 16 seeds and wound up with only six good viable plants. The ones I kept have grown much slower that I have ever seen. As I said above, since 4/20 they are only 24" tall. I use Fox Farms nutrients. I have looked at hundreds of pictures and have not seen any that exactly match what I have. The plants and the leaves look OK but there is a definitely a yellow cast to the leaves to them. I would greatly appreciate any insight that any of the growers here can offer. Thank you.

 

Here's a picture.