Injured Glowlight Tetra

With an injury that hasn't become infected, lots and lots of clean water are the best option - that is, a lot of water changes. Do you have a quarantine tank you could put it in till it is starting to get over the injury? Or even one of those nets they sell for fry? Leaving it in the tank with the other fish makes it easier for the others to pick on it which could delay any recovery.
If it has become infected you would need to treat the infection, something best done in a separate tank to avoid exposing to other fish to medication unnecessarily.





If it comes to the worst, do you know how to euthanase fish?

For futture reference - hopefully you won't need it - you are quite right thinking putting the fish in the freezer is not a good idea. It is a slow death.

For small fish, you can put some water in the freezer and when it forms a layer of ice on the surface, break a hole and drop the fish in. This is OK for small fish like glowlight tetras but I wouldn't use it for, say, a betta sized fish.
The other method is clove oil, which you buy from a pharmacy - it's an old remedy for toothache. Put the fish in a tub of tankwater. Shake 25 to 30 drops of clove oil into a bottle, add water and shake till the oil has emulsified. Then pour into the tub with the fish in. This can somethimes just knock fish out so make sure they really are dead before disposal. Warning - clove oil stinks so you won't be able to use the tub or bottle for anything else, and the smell takes ages to wash off your hands, so I use rubber gloves.