Today's the day.
I was doing some research, looking at valid reasons for and against pet euthanasia, and two valid ones that applied to Ronan were “the animal is suffering from a terminal illness that medical or surgical therapy can no longer relieve or help” and “the pet has a severe, chronic disease where death from the disease itself is unlikely, but drugs are no longer helping the pet with its pain or mobility”.
The article talked about quality of life. Ronan was mostly resting on the substrate, rarely, if ever getting up to swim like Magnus does. He paled out, showed stress stripes, lost weight, and struggled to swim correctly. He was nowhere near where he used to be, as far as life and vigor goes.
Here's some pictures. I'm going to do a water change, and siphon heavily on his side of the tank before letting Magnus have the full aquarium.
I’m sad I had him for such a short time, and that most of his life was in an inadequate habitat. At least he’s not in pain anymore.
He'll be buried in my garden.