A little concerned

Growing four ladies. They are in week four of flower. Three of them seem to be doing great but one seems slower then others and has some leaf issues. I grow in coco with ph in five. I use the drops so I don't have an exact ph but I try to stay on five/ six. I figure I need to buy a ph meter. ( any suggestions). The leaves are brown around the edge, crispy and now seems like spots. PPM was 720. Like I said though the other three don't really have this.
any advice?

Does that one plant have more Sativa in it than the others?
My Sativas always want more nutes than my Indicas.
Try to stay closer to 6 with your PH, you want 5.8 really, but try to go up a little anyways.View attachment 2644193

^^This^^ X 10
I'm guessing bag seed ?
Mine is and all three are totally different beast. AKA Lovely's.
Every pheno has a "Tad" different needs.

It's a clone and yes its sativa but idk more then the other ones. from the pic above it looks like potassium def. Anyone know a product that will help? I also need a ph pen for accuracy.