Hey there, unfortunately my friend this is magnesium deficiency. This you can see from the yellowing on the sides of the leaf. This is not due to the nutrients actually but the light schedule. 18hs means the plant goes in photosynthesis "overdrive" meaning it needs more magnesium per day than normal. overtime you find a deficiency. To stop the problem from spreading add calmag more regularly as well as lower the light schedule as much as you're comfortable with. Remember : THC and Terpenes degrade with light meaning less light often means more potent product 😉 Hope this helps 🚀