brown spots and curled tips

Hello. Ms. Grower. Do you have little black fly's flying around? Fungus Gnats look harmless but can do a lot of damage like I see in your photo. Cinnamon will kill the fungus that they live on. Just spread some on the soil and water in. This is my first thought. Hashy is right: at this stage in the grow you need to cut down on the N with more P and K. Yes I think you are right with K deficiency. My only concern is, autos use about 1/2 of the nutrients that photoperiod plant do. So don't add too much and burn them. With the new LED lights you need a bit more Cal-mag than you did with HID lights. Your plants look healthy otherwise. Good Luck. Chuck.

Didn't look at quantity of nutes you was putting in. Theia is right way to much.

If you are putting over 6ml per litre of calmag per feed into soil as your diary shows I think that's way too much..dial that back ...

I wouldn't worry 2 much about it. Just start lowering your grow nutes as she is going into flower and increasing your bloom, keep cal mag at a low dose. Good luck.