A few times over my fishkeeping life I have had to put some of my favorite fish buddies to rest. I have heard of many ways to safely and painlessly euthanise fish, but I think everyone knows in reality it's never a fun thing to see your long time companioin writhing in pain. I found this little article today that let me feel a little bit better...
http://www.smh.com.au/articles/2003/02/10/1044725683181.html
http://www.smh.com.au/articles/2003/02/10/1044725683181.html