Can anyone tell me what deficiency this has?

Growing in dirt.
Water twice a week, once with Advance Nuetrients and once with straight water.
Been flowering for about 3 or 4 weeks, and all the plants around it (same water schedule and light) look fine, this is the only one that looks this way.
Sits under 2 600w HPS lights.
Ph meter is broken, so I'm not sure of that, but like I said, the rest of the plants are fine.
I also posted this under the General Growing sub forum neutrients and someone said Potassium. What can I get that from? Chop up a frickin' banana?
Any help would be appreciated, I'm a noob so be gentle.

They won't all be perfect man, I would say if the others look fine then you aren't doing anything wrong. More than likely this plant just doesn't like you

the purple leaves could be just be symtoms of a purple strain but the mottled green/yellow/beige/purple normally signify a toxic salt build-up, flush them through with 3x the volume of the pot.

x2 for buildup

tbh..looks like it has just about all of them lol. i looked at a little guid chart with pics and that looks like its got a little bit of every nutrient deficiency.

if i where you id get a nute that has a little bit of everything in it lol. maybe something like MG really. they are good for that. all purpose has a little bit of alot of good nutrients in it that your plant is probably lacking.