The least gruesome way to kill a tropical fish is to put it in a bucket of tank water and pop it in the freezer. As the temperature drops the fish slows down and become unconscious and eventually dies. If you can add some clove oil to the water it puts the fish to sleep and it is asleep while the water gets cold.
However, if they all have it, you might as well try to treat them. Clean the tank and add heaps of salt. Add the anti-biotics when you get them and hope for the best. You can use salt and anti-biotics together.