[Keri]

Clove oil
Add a couple of drops to the bag w/ water and fish, puts them to sleep (it will be sleeping within minutes of dosing the clove oil) Then however you kill it it won't feel.

edit: here, i found a link with good directions:
(from wisegeeks)
There comes a time in every aquarist's life when a fish that can no longer recover from disease or injury must be euthanized. This is no easy task but it can be done humanely and peacefully without stress to fish or aquarist.

The best method is a two-step process. First, anesthetize the fish with clove oil so that it is sleeping and unable to feel pain; then introduce a clear grain alcohol like vodka to ensure the fish will not wake up.

This method is commonly misrepresented as mixing clove oil and vodka together. That is incorrect. Clove oil must be introduced first, allowing the fish to fall asleep before introducing vodka. Vodka will be stressful for a fish that is not anesthetized.

Clove oil (eugenol) is available at any drug store and is sold as a toothache remedy. It has been used for years as a fish anesthetic for surgeries and tagging procedures. Clove oil will put a fish to sleep and ensure it feels no pain. However the fish can wake up from this sleep if removed from the clove bath. The last step of adding the vodka will ensure the fish expires.

Here are the steps for fish up to 3" (7.6cm) in length:

1. Add tank water to a measuring cup or mixing bowl. Measure the amount of tank water you add to the cup or bowl and make a note of it. Place the fish in the container. If the fish is in a clear cup place a dark towel around the cup to calm the fish.

2. Fill a small, clean jar or bottle with tank water, leaving some room at the top. This might be a baby food jar or pill bottle. Put 1 drop of clove oil in the jar or bottle, cap and shake vigorously. The clove oil must emulsify, turning the water milky white.

Gently pour about 1/4 of this emulsified mixture into the fish's container. The fish will begin listing as it starts to fall asleep. Let the fish be for about 10 minutes. The fish should be resting on the bottom by then. It will look dead, but if you watch closely its gills will be breathing once every few seconds. If after 10 minutes the fish is still rising off the bottom swimming intermittently, retrieve the jar or bottle of emulsified clove oil, re-shake, and add the same dose to the fish's container. Wait again.

3. Once the fish is asleep on the bottom, add 20-25% white grain alcohol. For example, if the fish is in 8oz (240ml) of water, add 2oz (60ml) of vodka. Let the fish stay there for at least 20 minutes.

4. Check the fish carefully after 20 minutes for any gill movement. If there is no gill movement over a 60 second period, the fish has expired.

For large fish: Place the fish in a bucket or plastic tub with tank water. Again, measure how much tank water is used. The dose for the mixture in the jar will be 10 drops of clove oil per gallon (3.78 liters). For example, if the fish is placed in 3 gallons (11 liters) of tank water, fill your jar with tank water and add 10 x 3 = 30 drops of clove oil. After shaking the jar vigorously, slowly add the entire mixture to the bucket or tub that contains the fish. Gently mix it in. Once the fish is asleep, follow the previous instructions for adding 20-25% vodka.

To eliminate vodka from the procedure and overdose with clove oil alone: Put the fish to sleep first as stated above, waiting 10 minutes for the fish to settle on the bottom. A lethal overdose of clove oil is 50 drops per gallon (3.78 liters), or 5x stronger than the initial dose that anesthetizes the fish. Using the same example, if the fish is in 3 gallons (11 liters) of tank water, the jar's lethal mixture will include 50 x 3 = 150 drops of clove oil. Administer it the same way, by first emulsifying (shaking) the jar before adding the mixture to the fish's container.

Once you've added the entire mixture to the fish's container, wait a few hours. Finally, make sure there is absolutely no gill movement by watching the gills closely for at least 60 straight seconds. If you see any gill movement, add more emulsified clove oil.

Using clove oil alone is not recommended because even though a fish looks dead it can recover once it has been removed from the bath. Clove oil is a preferred anesthetic precisely because it is hard to overdose a fish with clove oil. Therefore be especially diligent when using clove oil alone that the fish is really dead. It is much safer to use vodka as the final step.

Unacceptable methods of euthanasia are: freezing, boiling, chopping, removing the fish from water, using a seltzer tablet, slamming, pithing, decapitating, or flushing down the toilet. These methods are slow, torturous, stressful or violent. Clove oil followed by vodka is both inexpensive and humane. The fish goes to sleep like we might before an operation, and simply doesn't wake up.

Hopefully you will rarely have to perform this task, but when you do, it's at least comforting to know your fish does not have to suffer.