Can someone tell me what this is from?

[25hewitt17]

I feed them in the 11th and tested the runoff and it was red (low) so I fixed it and now it is yellow .....could that of been what caused it? I don't know

 

[Matthewstever]

Could be...pH issues make em do weird stuff.
I used color strips, then liquid, and always had issues. I got a $13 digital meter from.amazon and my pH issues went away...

 

[Jimster]

What medium and what type of fertilizer are you using? The plants don't look too bad to me and I wouldn't worry about the few brown spots... they could be caused by anything weeks ago, so unless it starts spreading or growing bigger, I wouldn't worry too much. I use Promix, general 20-20-20 fertilizer,and feed every 12-15 days once the plant is growing well and haven't had any Ph issues or nute overdoses. A little more info is needed, but things look pretty good overall... at least in my opinion.
I looks like something is hacking off the ends of the leaves, though....