Is this a zinc def or K?

All at top? Zn, if you've deduced it to these 2 possiblities. Your in soil, so bring that pH up closer to 7. You want it slgihtly acidic, though, so err on that side of 7 in soil. ~6.5-6.8. It's minimal, so it shouldn't impact harves in a noticeable way to your eye... maybe even within normal variance statistically too.

Hello If you're watering with tap water you usually shouldn't wind up with a zinc deficiency. Id suggest raising your ph just slightly higher into the 6.3-6.5 range and I think the zinc will become more available to the plant. Happy flowering