Sorry to hear it, man but mad-props for giving it a good life.....when you lose a fish that old it's like losing a dog.
I personally do not believe in freezing. That process puts fish through clear distress. It sucks but it's my obligation (in my opinion) to do it quickly and painlessly. I use the one hit method. Heavy on the owners heart but it's instant for the animal.
this is the 1st time I have seen this posted on here, and I always shunned the thought of clove oil or deep-freeze. I was just fishing Sunday w/ a friend and had a Green Sunfish I was using for bait and generally they are so hardy I can remove them afterwards, put them back in the livewell w/ salt and stress coat and they heal up for next time or just release them, however this one got battered into the rocks and wasn't going to make it, so when I realized he was a goner, I didn't want to drag it out and let him suffer so I sent him head-first into a rock.
Brutal, but I have your back in saying it's MUCH quicker and MUCH more humane to make it quick. I used to do the same w/ my rodents when I couldn't find any pre-killed-frozen.......grab them by the tail, whack the head on the ground, then submerge them in water while unconscious. Dry ice is cruel and so is cutting their throats. Places that use animals for research for cancer and diseases do NOT practice humane way of dispatching test animals......I saw one girl drain a Rooster of blood to the brink of death and simply tossed it in a trash bag while it was still alive and breathing.
For God's sake, make it quick and painless if you have euthanize.
R.I.P. to all the animals who died in our care.