I've never heard of the Magic Disease Treater medicine. What your fish are suffering from is extreme-over-crowded conditions and poor water quality, in my opinion. No matic medicine will cure them until the space and water quality are addressed.
Not sure what you mean about not knowing about ammona, nitrite and nitrate as you are not a professional, but test kits are available to any fish keeper. Perhaps your fish store can test your water for you.