What is happening to my plant?

[GannerUK]

Hi all, new to growing and this site. I'm about week 5/6 of flower on a random bagseed, I'm growing in coco 70/30 mix and i'm feeding a litre of tap water mixed with Canna A & B and have been for the entire grow near enough. recently i noticed these patches on 3 or 4 leaves towards the top of the plant. Someone please help me as i have no clue what's going on and i don't want my first girl to die because of my ignorance. ignore the holes and stuff that was from when it was outside ages ago and a bug got to it.

 

[GannerUK]

couple more pics

 

[WhatsHeDoingToday]

Potassium deficiency. Potassium may be present in soil but may be locked in by high salinity. Leach the toxic salt out of the soil and then apply a complete N-P-K ferilizer. This picture looks like later stage of Potassium (K) deficiency. Pg 251 Marijuana Horiculture- The Indoor/Outdoor Medical Growers Bible. By Jorge Cervantes.