What is this?? Southern Ontario Outdoor Bubblegum Grow in Pots...

I've got 5 bubblegum plants growing outdoors in Southern Ontario. They're in 5 gallon pots with Promix soil. I've been using Fox Farm, but only the Grow Big and Big Bloom (at half strength every other watering, which means 1 a week). For the most part, everything is fine - except this is happening to one of the plants. The one plant in question seems to be a runt compared to her sisters, which are doing beautifully.
I can't seem to identify this online, I've tried and found some similar things but not quite.

Any ideas?

Almost looks like low nitrogen or early calcium deficiency. I would lean towards calcium because of the brown tips and spotting. I also use FF nutes and have to supply the calcium from other sources. I use 8-10 old egg shells boiled in water for 15 mins with a tblsp of epsom salt for magnesium. Homemade CAL-MAG! Make sure your PH is good before you do this though or it won't make any difference due to nute lockout. Hope this helps.

Keep it smoking!
D

Thanks for the reply, I will try that...
I adjust my tap water with about a teaspoon of vinnegar per gallon. As you can see the other plants, all the same strain, watering, feeding, soil, pots, nutes is exactly the same and none of them have this problem.