Looks like Potassium deficiency to me.
The plant on the left is starting to yellow at the edges too, it appears. I'm experiencing that problem with my Strawberries and lettuce. I added 5ml of Potassium Nitrate to my reservoir.
I have 80 litres (21.13 US Gal.) of nutrient in the res.

Glen

Whew! I have been searching for this answer ALL morning! We have a large (24 site) homemade system with different veg in it, all of which require the same-ish PPM and pH. But the leaves of our zuchinni (I know, I know you shouldn't grow zuchinni hydroponically, but we are not novice or new growers) and cucumbers have began to do the same thing....turning yellow right along the edges....but with all the research I've been doing, it was either deficient in magnesium or potassium or too much nutrients. Tks guys and good luck!