This plant is three weeks into flowering and starting about a week ago I noticed some yellowing leaves lower down on the plant. The leaves are turning yellow and developing brown spots. The tips are also curling up somewhat. Since this started appearing on leaves lower down I thought it might just be old leaves dying off but it has spread to about halfway up the plant. The plant also hasn't stretched much after starting flowering and a friend's plant that it was cloned from stretched a lot. After some research I thought it might be a calcium deficiency but I'm very unsure and would like an opinion of someone more experienced. Some additional details: Plant is being grown indoors in soil under a 400W HPS light that is about 50cm away. It is watered with tap water that has its pH adjusted to around 6.5. It is being fertilized with Canna Terra Flores which has an NPK of 2-1-3. Temperature is usually around 25C but can drop down to around 15C or lower at night when the light is off. This plant was grown from a cutting rooted 7.5 weeks ago and has been in flowering for 3 weeks. Strain is "Zombie Kush". Let me know if you need any more details or pictures. Thanks.