[HeyPK]

it looks like severe potassium deficiency. Are older leaves on any of the other plants diying back?

 

[HeyPK]

On second thought, it might be pleco damage. I see that the tip of the leaf is relatively undamaged. If it were potassium deficiency, the leaf would die back from the tip and from the sides. This damage looks to be centered around the midrib with the sides and tip in better shape. I bet it is your big bristle nose catfish scraping the leaf. Don't know what you can do about that. Maybe give them some zuccini.