How to fix yellow leaves, brown spots.

You must input all the data about how you cultivate, such as pH, watering, nutrients, EC, temperature, extraction. Everything matters, with that you can start getting an idea of what's happening with your plants

As they already told you, it's calcium and magnesium.. I recommend you remove the most damaged leaves and supplement for at least a week, otherwise you risk ruining all the leaves, which translates to low photosynthesis and small buds. If you supplement a bit, you won't have problems. If you have time, check out my diaries; I often have this issue with the bigger plants...(I use coco, so I already use it normally)

The old leaves on the bottom turning yellow is normal. As the plant grows bigger the less light the leaves on the bottom get as for the brown spots you probably have Calcium deficiency and should adjust the amount you're giving them.

From what I can see you have a Sulphur deficiency and could rectify it with a good dose of Epson salt at a tsp per 3.6L if you have manure that would also be a good source as well...Nothing serious just a minor hiccup, they'll recover nicely...

You need to measure ppm of the solution you’re giving and runoff as well. At this stage/size of the plant ppm 600-700 is good. Runoff ppm lower than inflow most likely indicate underfeeding. Feed more frequently to reduce the hard impact of high ppm when feeding every 3 waterings. That is probably the cause of the problem. Feeding every other or each watering is much better. So start slowly. Raising inflow ppm not more than 100 at once in case you need to increase is safe for the roots and the plant overall. pH is important, but there’s no actual need to measure pH of the soil. Inflow/runoff pH can give you enough info.