Hey there,
Don't buy any more fish until you have found out what the proplem(s) are.
Carbon filter media will suck out the meds - so remove while treating fish.
There has been a lot of debate going on abot the best way to euthanase (sp?) a fish. I think we have decided that freezing a fish is painful (to the fish) because ice crystals form in the fish's bloodstream before they lose conscious. I think we also have decided that uh whacking the poor fishy on concrete etc. or putting a sharp knife through its spine just behind the head are two acceptible ways of euthanising fish. Others will, of course, disagree with me.
Also, as Sandy has said, how often and how big are your water changes?
To get more oxygen into the water you need to have a fair amount of surface area and/or disturbance. Air bubbles, from an air stone, don't actually directly affect the oxygen levels in the water.
HTH
George