that couuld be a lot of things broham. could mean you need some magnesium. if you use tap water i doubt its this because the water from your tap should have enough mg in it plus the nutes you use probably have a little too.

could mean you have spidermites (thats easy to check just look underneath the leaves). i dont know about the mites though because only that one leaf is being super manhandled. if you do have spidermites though might i recommend a hot shot no pest strip. it will take care of all of them easily, quickly, and without a mess. just hang it up and theyre gone within a couple days. on anothr note i feel like im always saying this haha. hot shot should pay me some royalties.

it could mean the radiant heat around your leaves is too much. are you using cfls around the bottom of your plant?

it could also mean you have a fluctuating pH, i bet due to a combination of possibly overferting combined with the salts fox farm leaves behind.

good luck!