So everyone says if you're growing in hydro mediums you need a pH pen. I was always like "yeah, I'm sure I can get by with pH strips". Finally after mixing a few batches of nutes and guessing that the pH strip was measuring between 5 and 6, I thought I was fine, but decided to get a pen as it seemed faster and more accurate.
Well....I tested my current mix of nutes and they were at 7.6. Yikes! So glad I found out early on in my grow.
Moral of the story is, you need a pH pen if you're growing in hydro. Get a pen.