Magnesium deficiency. If your brown spotting starts off as golden yellow spots that are small and numerous, then it's mg def. This can be solved with some epsom salts if you want to fix on the cheap. One half teaspoon of salts in a gallon of water and give that next time you water. The fix takes a couple of days to disappear. For alot of growers I usally recommed that instead of miracle grow that people try a fert called Dynagro as it has pretty much everything plants need, including calcium and mg. It just doesn't have a very long shelf life, maybe six months. For now I'd probably try the epsom salts and see what happens. I wouldn't necessarily try to switch soils or even transplant if you don't have space, but if I were you I'd plan on taking some clones of good branches and start them in small cups of soil. Cloning is an art in itself and everyone should know how. For the future I'd probably use Foxfarm fert and throw in one half cup of their Fruit and Flower powder fert that the grow stores sell. Mix one half cup or so for every 2-3 gallons of soil and you probably won't have any more problems. Some people say these ferts don't work, but if that's the case why does the company make them? Mix the fert into the soil and just water when dry. Easy. Oh you might switch to 5 gallon buckets in the future.