Help! Anyone know what this is?

I am growing 5 plants in a 4x4 and the largest of them starts to have this problem. The rest are also showing this problem and they are starting now. Do anyone have any idea of what is causing this ? The temperature is 24degrees, humidity is at 70%. Largest is on flowering and the rest is on vegetative. Using DWC system with general hydroponics nutrients. Can anyone help me to figure out what problem I'm facing right now?

Temperature:24

Too much N not enough P K

Looks like potassium issue. As mentioned above.

That my friend is a sickly looking marijuna leaf. Have you had it tested for COVID?