I'm gonna guess mag only because I just had a similar situation. JmoView attachment 66296potassium deficiency?
I would guess your ph is off. I don't grow in soil but I have 4 in solo cups right now. First batch of water was phed right and they grew good. Second batch of water was middle 4s for ph when I realized my leaves were turning color just like yours. I adjusted ph of that water and flushed the cups with it. 2 days later and they are not looking any worse.View attachment 66296potassium deficiency?