Its approved use in the US is for anasthesia and sedation only.  It is often used in commercial fisheries for things that would be too stressful to fish - eg: taking them out of water for collecting eggs or for first aid etc.  It can even be used as a light seditive for transporting fish.

Depending on the concentration in the water, the effect can be incremental though, and they go into deeper and deeper anasthesia over time as the gills absorb more until it matches the blood stream concentration.

And that's where the euthanasia aspect comes in.  They first go into light anasthesia (loss of flotation control, effectively they're unconscious at this point), then Deep Anasthesia (complete loss of all reflexes, eg: suitable for surgery) and finally, if the level increases even more they stop "breathing".  After at least 10 minutes they should be fully "gone".

Some fish species have adapted for low oxygen water and can apparently survive without oxygen for a while, hence the 10+ minutes wait.  Or a secondary physical euthanasia method is required to ensure no suffering.

I've noticed that lots of universities here in the US specify MS-222 for pre-dissection protocols...

I do appreciate your efforts to find the gentlest way to euthanize seriously ill and deformed fish.

You probably have read that one shouldn't breath it. Some authorities suggest one shouldn't even touch it. We do have a pack of surgical gloves around here for various household and fishy purposes. That might not be a bad idea for you, but ought to really impress your wife!

I'm sure you are aware of the need to carefully and responsibly dispose of that stuff.

"Another site mentions that a 0.5ml scoop holds about 400mg of the powder and that can be directly dissolved in about a liter of water."

I would imagine chemistry equipment sites could offer appropriate measuring equipment. One could size even smaller, if not precisely small quantities with half full scoops.

Like miskairal, we also picked up a little vial of the clove oil at a local drug store for $6-7. The package is so darn small, it is easy to lose on a shelf of aquarium stuff.

Had a hunch it was pretty potent stuff and not to be used in the fishroom. Thank you miskairal for the advice to use it a long way from the fish and probably outside.

I was surprised when Terry Fairfield, last Friday, endorsed putting an ailing aquarium fish in a jar of water and placing it in the freezer. Maybe he feels that, for most aquarists, it is the most accessible, mostly humane approach, to euthanasia  and a vast improvement over flushing unwanted fish.

I really liked his observation that fish are not disposable pets.