The kindest way to put him down would be to put him in a little tub with his tank water, and stick it in the freezer. His metabolism will just fall and he will fall asleep peacefully. That is what I have been taught anyway, may want a second opinion on that.
I am so sorry.
Freezing is actually the worst method, the fish slowly gets ice crystals in the bloodstream, until the heart cant process it anymore, the fish then has what is best described has heart attacks and dies a slow painful death.
I usually go for the swift blow to the head
Sorry to Hear bout fantasia.
As DarkEntity has said, Joshy's Meathod is the worst way it is slow and painful, same as you sitting in the frezzer getting colder and colder till you cant tank it which is a long time, is i mentioned in another post if you have no colve out or unabe to sever the spine if you freeze some water so there is a thin layer of ice on the surface, break the top layer so the ice cold water is expesed then put the fish into the water, the sudden temp shock to the system shuts the organs and nrves down almost instantaniously.
Scott
If he doesn't pull through id suggest cutting behind his gills from the spine to pectoral fins, I know it sounds harsh but it's quick and less painful for the fish but i guess not for you