General Hydroponics Ph Test Kit, 1 oz

That would be my guess. My cucumber leaves started drying and I found out the pH was down to 4.0 which is way too low. It's worth getting a test kit, they are pretty cheap. This is a good option: https://a.co/d/00IM0O7B On the same listing you can get the full kit for about $11 more. You'll need the pH Up and Down if you find your pH is off. It's cheaper to get the kit than if you buy it all separately.

I gave up on ph meters they either break easy or calibrate incorrectly, I just use TDS/EC to the ppms. I use ph strips or drops https://a.co/d/0dpJSG0t https://a.co/d/08fK3XA6 My ph has been dialed in using low amount of nutrients and cal-mag complete to drop the ph 5.8-6.2 prepping my water because i grow coco and If i need to increase the ph i just add a pinch of baking soda. I vegged some plants for 7 weeks and then flipped to flower last week and not had one yellow leaf or tip so far.