Wait let me understand.. you added Maracyn Plus meds to his tank ONCE and that was it? That would not have done a thing. The bottle says you're supposed to add a fresh dose not premixed every other day.. so he should have had 6 doses in 12 days.. Also has he been in that little cup for 12 days? 10ml of stress coat would be for a 10g tank.. If he's still in that 2.5g tank he should be getting a MAX of 2.5ml of stress coat.
To mix up meds read how much one dose is for. For example, if one package treats 10 gallons, take 10 teaspoons of tank water and put the whole package in it. Mix it up well until it's dissolved, then take as many teaspoons and gallons of water he's in.. so if he's in a 2.5g you take 2.5 teaspoons of the mixture and place it in his tank. You have to remix everytime you dose.