Tony, are the fish just dark or do they have any bacterial build up or parasite externally? I had a discus turn dark recently with no external symptoms but with fast breathing. I treated with 1.5 times dosage of Metro twice a day for a week and it recovered. I put in epsom salt with it as well at 1 tablespoon per 5 gallon. i also bumped temp to 91 degrees and only changed water once a day (so 2 metro doses per water change).
Also when I do salt baths for external infections I do 3 tablespoons per gallon. I just watch the fish to make sure its not turning sideways. If it does I keep adding 1 cup of water from the tank at a time to dilute it a bit until fish swims upright. Never lost a fish from salt dips using this method. Some fishes handle 3 tablespoon per gallon fine without having to add any water.
Good luck.