Calcium deficiency.
Add cal/mag to your line up or increase the dosage if already using a cal/mag supplement.
Remember too, what is showing now in the leaves, is a direct reflection of what was happening in the root zone 2-3 weeks ago, meaning this issue has been manifesting for some time already.
Also, improvements will take 10 days or so to start showing up, it won't happen overnight!
But the sooner you start, the sooner you prevent the issue from becoming worse.
Damaged leaves will stay damaged unfortunately, but there is no reason to remove them just because they "look ugly".