I was going to rec the brine shrimp, but you've seen that a dozen times now. That being said, when people get to that point of old-age, they just aren't hungry anymore. Their taste buds and sense of smell have dwindled over the years, and they just get sleepier and sleepier. They're not in pain from hunger, they just get really, really tired and they sleep more and more until, at some point, they just don't wake up. Same with horses/dogs/cats, I'd assume just about any animal.
So, I doubt that he's suffering, I think he's just getting to the point where he's ready to fall asleep and not wake up. He's apparently had a fantastic life with you to have lived such a long time, and he's ready to swim home. :)
And I completely agree with Ayala, there is no such thing as "just a fish" or "just a dog" or JUST anything. They are living, breathing, thinking, feeling creatures, and they are loved and love in return. That is not JUST anything, any more than a life is JUST a bunch of chemical processes. Anytime there is love and care shared between two creatures, that is a special, special thing, and it is valuable and very hard to lose. There should NEVER be any shame in feeling for a creature, not ever. This is what makes us whole.
I know that he knows that you are with him, allowing him to go in his own time and caring about him for every minute that he continues on. I wouldn't euthanize him at this point, he's ready to go, he's just got to figure out how.
Thoughts and prayers and internet hugs for you. :(