leaves brown stains

Looks like Phosphorous to me. Did it start on the lower older leaves and progress up the plant? The brown patches very much resemble Phosphorus issues. The question is, the cause? Look how reddish your stems are. That's another indication of Phosphorus deficiency. Phosphorus deficiency displays very similarly to what your plants are displaying man. I don't know what your feed regiment is, or how often you are feeding and what the macro's are, but Either you aren't feeding them enough, or there is enough but it is locked out due to some factor. I'd be curious to know what the plants main stems look like. Are they green or are they reddish as well. If your main stalk is red too you are Phosphorus deficient almost certainly. Leaf stems can be red on almost any strain as they get for lack of a better word, "tanned" if you will, from the light. It's a common genetic trait as well. Reddish stalks generally aren't though. Your temps and humidity look good. But if you want to have the best environment for your plants you really should concentrate on maintaining VPD. It will be the best thing you can do for your plants to ensure proper uniform uptake of nutrients. Review your feeding schedule and mix rates. If you feel it's not enough, then up it. If you feel it's plenty, then flush your medium with pH'd tap water so there is plenty of runoff. You want to flush out the nutrient salts that are in your medium which could be creating a lock out. Then allow your soil to dry out so the pot feels light. Then re-feed with proper pH solution.