Is it time? And how to humanely euthanise?

Hi there...

sadly, I am faced with the hard decision to possibly humanely euthanize my betta fish.

Ted has been with me for roughly 10 months, but since he's from a pet shop (there's no responsible breeders here sadly) I don't exactly know his age. He developed a tumor quickly after I got him and it's progressively growing bigger. I never knew about this dragonscale problem, but I hoped to provide a good life to him after I learned about the melanoma thing.

Heartbreakingly, I found him to be resting more and more over the last few days and today I found him unresponsive on his side for the first time. What I hoped was a good nap, turned out to not be death, but he definitely struggled to get up. It seems like the melanoma has attacked his back end and he's slowly losing function in his hind. He didn't want food today, either. He swam up to it, took a breath, watched the food and eventually went back to lie down after a few seconds. I checked temperature and water, both should be fine, so it's not acute poisoning.

I don't want him to suffer. As much as I love this fish dearly, I want to grant him an end in as much peace as possible, if needed. But I don't know how, and if his time has really come? I can't hit him with something, I'm afraid I'd hesitate and prolong his death this way.

Is there another humane end for a betta fish? Would you say rather sooner than later? I'm torn. Photo one is him lying down after the unsuccessful feeding try, the second and third show his melanoma a little better I hope.

(Ignore my dirty tank glass, it needs cleaning since I couldn't stand long enough after a surgery to do more than water changes. Especially behind the filter.)