This looks like a root disease or nutritional problem to me, possibly too alkaline soil? I'd be upending that and having a look, then transferring it to a container with fresh rich slightly acidic say ph 6 potting soil. It's stunted and has a bit of necrosis in older leaves....hope this helps.

It's probably the alkaline soil then. I'll fertilize it.

for the advice!

I'm sorry I missed this... possibly I'm too late, but fertilizing it wound help that much, might even stress it too much, I'd recommend miracid.

I have a soil acidifier made by Espoma. Would that work?