Considering your environment, it sounds like a pH issue. Spots are usually a miconutrient deficiency of some kind. Yellowing is more commonly a major nutrient deficiency. 9 times out of 10 it's usually a pH issue. You need to test the pH of the runoff water and make sure it's inbetween 6-7.
A great product to correct micronutrient deficiency is kelp. Algamin was one of my favorites for soil... But if your water source is too high or low in pH even kelp won't be able to help because it won't be able to absorb through the roots.
You can try foliar spraying them with a mixture of kelp and water without any clorine (sp?) use r/o or distilled water and some sort of kelp-based product.
You have to know what the pH of your runoff water is first though. Your environment sounds great- it might be your soil nutrients, or the pH of the soil at this point. The humidity is also quite low, but it's more likely a pH issue.
If the pH is fine then the soil doesn't have enough micronutrients. You'll need to feed them very lightly and add some sort of kelp.
A very inexpensive product to try to help them out of the slump is SuperThrive. I swear by the stuff when I have a plant that is under the weather!
Good luck brother!
