Calcium deficiency.
Remember too, what is showing in the leaves now, is a direct reflection of what was happening in the root zone 2-3 weeks ago.
I would say this problem has been manifesting itself for 3-5 weeks already, it is quite advanced.
Adding a Cal/Mag supplement to your feeding schedule or upping the dosage if you are already using one will help.
The damaged leaves will not recover, but it will slow it from getting worse and eventually stop once the plant is getting adequate calcium again.
You can be quite generous with the remedial dosages (double strength), it is hard to over dose with cal/mag, but settle back down to normal dosage recommendations once yiou start seeing improvements in the plant.