I would euthanise because when he does die (and it sounds like he's going to unfortunately) the other fish will eat the body, and if it is some sort of disease that's killed him they can get it too. This is how fish TB, in particular, infects and you want to avoid an epidemic of that at all costs.
Clove oil is very humane, failing that, half vodka half water in the freezer for an hour or two and then drop the fish in. I usually don't recommend freezing of fish, but for a fish as small as a glowlight he will die within seconds. Clove oil takes a while but is not painful at all, it's an overdose of anaesthetic.
What sort of flakes are you feeding them? Cheap flakes often use plant proteins, which are not complete - these foods will list alfalfa, wheat flour, rice/bran or a legume based food in the first few ingredients. Foods where the first few ingredients are fish or meat products (fish meal, shrimp meal, meat byproducts) contain more complete sources of protein. Both plant and animal based proteins will read on the label that it has somewhere between 30 and 40% protein usually, but this protein should be mostly from animal sources.
Spirulina in the ingredients is a good thing, it's very good for fish. Be careful of anything that claims to be colour enhancing, especially if it claims to enhance red colour. These mostly contain large amounts of astaxanthin, which means they will turn your fish orange. NewLife Spectrum does seem to enhance colour without containing ingredients that are basically dyes. That's what I feed, more because of the improved vitality of the fish than the improved colour, but I have to say they look better.
The other thing about flakes is that if you dump them all in at once, by the time the fish get to the last ones most of the nutrition has leached out into the water. If you are feeding flakes and not pellets, it's better to sprinkle in just a few at a time so the flakes only sit there for a few seconds before the fish eat them.
