If you think it would be kindest to euthanize him, I have a suggestion for a very peaceful way out. I used it when my Goldfish, Bronwin, needed to be put down. He had a jaw deformity and was starving because he couldn't open his mouth. Developed dropsy, and was in a lot of pain.
I don't know what size tank your fellow is on, or if other fish are in with him, so I'll just suggest this:
- Gently scoop your fish up into a fairly small container filled half way with water from the tank.
- Get a bottle of clove oil - the kind for analgesia/dental numbing, not the scented oil. I've heard varying bits of info on the dosage, but the bottle is usually small, so I've always used the whole thing to make sure the animal is truly dead.
- Scoop up enough tank water in a separate container to fill the rest of the one your goldfish is in. Place the clove oil into this container and shake very thoroughly so that the oil is thoroughly dispersed throughout the water.
- Pour the water with added clove oil into the container your fish is in. You should see an almost-immediate cessationo of movement, and not long after, the gills should stop.
- Leave the fish in this container until you are sure it is dead. Most people will freeze the fish once it is unconcious, but I've found that if you leave them in, you'll be able to tell in a few hours that they are definately dead; the eyes will go pale and glazed, the color will leave, and the slime coat will shed. If you've seen a dead fish, you'll know what to look for.
- If you'd like to be doubly sure, freeze or destroy the brain after the fish has been in the clove oil for several hours. It shouldn't be necesarry, but some people like to do it on the off chance that the animal was only very convincingly anesthatized. But as I said above, if you just look for the signs of death, it shouldn't be needed.
Clove oil has natural anesthetic and analgesic effects on fish; they use chemicals from it when performing fish surgeries. It is not a painful death, and if you use enough and shake thoroughly before pouring it in, you won't need to use the vodka method, which sounds painful to me. It is probably the kindest way to say goodbye to your little fishie.
Sorry to hear he is nearing his end. The spreading red dot almost sounds like a hemmorage. Maybe the tumor as burst?
At any rate, look for signs of suffering, and chose what is best for you.