Anyone know what could be causing the leaves to do this and curl?

It’s just started this past week. It’s affecting mostly old growth on the plant. It’s starting at the side of the leaf and making them dry and curl inwards. At first I thought I got some of the leaves wet on one side of the plant but now it’s cropping up on the other.
Toxicity? Deficiency?

I get something like that at a low PH (under 5) in hydro. Iron and manganese are supposed to get real available to the plant and I'll get those "melty" looking spots on my leaves too.
It looks like you're not in hydro but how's your PH?