Yellow and brown spots on my plants.

This is my first grow, do you have any idea why my plants top leaves are yellowing with brown spots. The plant was planted in a 15 liter root pouch with BioBizz All Mix and 1,5 biotab. She is an auto flower and I used orgatrex and bactrex in veg stage. When she got into flowering I started using Bio PK 5-8, I have used that and some bactrex during flowering. She is in week 5 of flowering now. I did not ph my water but started doing that 2 weeks ago. To me it looks like a CalMag problem. She is growing under a SAN light 30w led light in a small box 35cm x 75 cm, the temps are around 26C at the top of the box and 24C at plant level, humidity is around 50%. What do you think it might be?

She's an Auto, 5 weeks in...don't try and throw the kitchen sink at her. You are correct in your observations, you're just a bit too far in to affect things too much. Where are you on your pH? How often are you feeding vs watering?

Yeah, I know I am a bit too far. I am just trying to avoid this in my next grow, I am going to ph the water from the get go next time. I water with ph 6.5 now, the tap water here is almost 8.2. I give her 1ml/liter PK 5-8 (https://www.biotabs.nl/bio-pk-5-8) with every watering, and I have only added bactrex once during flowering. I have a digital soil moisture meter I usually water when it is at around 20%. The buds still look pretty good and I think I will get a decent harvest, just trying to learn why she is turning yellow with brown spots so that I can avoid it next grow. I had no problems in veg and it seems like the leaves directly under the light is the worst affected.

I asked the guys at biotabs for their opinion and they think its too much fertilizer. The BioBizz All mix is very heavy fertilized plus I added 1,5 biotab to the soil. I will only feed her ph:ed water from now until harvest. Next time I will use light mix. I also think it has something to do with the grow space being small and they grow close to the led light, it is only the leaves closest to the light that show problems all other leaves and the nuggets that grow closer to the wall has no yellowing and brown spots.

How close was the light?

I agree it looks like the light was too close.
Did the top bud under the light look like a little piece of cauliflower at one point?