You can find accepted guidelines for euthanasia here:

http://www.nal.usda.gov/awic/pubs/Fishwelfare/iacuc.htm

The UCLA IACUC told me to use MS-222.

You may use either MS-222 or immerse them in ice-cold water (with ice).

You can use clove oil then ice-cold water and apply a procedure in Kumar, N.; Prabhu, P. A. J.; Pal, A. K.; Remya, S.; Aklakur, M.; Rana, R. S.; Gupta, S.; Raman, R. P.; Jadhao, S. B., 2011: Anti-oxidative and immuno-hematological status of Tilapia (Oreochromis mossambicus) during acute toxicity test of endosulfan. Pestic. Biochem. Physiol., 99, 45–52.  Generally, 25 mg/L clove oil is enough..

You can use clove oil at a concentration of 2 ml in 5 liters of water 

I will try the clove oil.

Thanks to all for their contribution!

I know that some researchers use and alcohol, but I am not sure is it human way or not?

Hi Mr. Nedic,

I don't think that the use of alcohol is a humane and/or safe way to anesthetize a fish. Currently, the only anesthetic approved for finfish in the US and europe is tricaine methanesulfonate (MS-222). In canada, metomidate is also available. Clove oil is used frequently by hobbyists and some retailers for sedation, anesthesia, and euthanasia. Furthermore, benzocaine is sometimes used for anesthetizing fish, although it is not currently approved for use. If you need a more detailed anesthetizing method or anesthetic descriptions, don't hesitate to contact with me.

Dear Mr. Salavati, thank you for the answer. I also use MS 222. Does it good for other small animals like amphibians?

Yes, it can be used for amphibians, but other drugs used in mammals can also be used in amphibians and reptiles (e.g., isoflurane, ketamine, propofol).

The best one is tricaine methane sulfonate (MS-222), you can also add sodium bicarbonate (NaHCO3) it's used to buffer the solution to a pH range of 6.5-7.5, since the Ms-222 decrease the pH of water. 

In the USA MS-222 is about the only method that will be approved by an Institutional Animal Care and Use Committee for euthanizing fish.  If you want more information on care and handling of fish for research see the book from NRC, "Guide for the care and use of laboratory animals, 8th edition."  Copies are available at the National Academies Press 

www.nap.edu

The safest chemical is tricaine methanesulfonate (MS-222)  for your case.

I use MS-222 for my zebrafish. MS-222 is also used for other fish species.

The best, cheapest, least dangerous and no effect on physiological parameters in fish is clove powder. Depending on the species, its value is different. For rainbow trout appropriate amount of 150 mg per liter.

Regarding amphibian, in addition, you can use alfaxalone, not the old presentation (ALFAXALONE - ALFADOLONE) but the new formulation: alfaxalone in cyclodextrin...good paper from Bauquier et al a year or 2 ago

MS-222 is the standard one, commonly used in our laboratories. But, I know, some are using CLOVE OIL instead.

Thanks for the answer!

clove oil is safe and cheaper

Best is tricaine methanesulfonate (MS-222).

Electrical shock maybe a good way.

MS_222

Clove oil can be used as an anaesthetic drug.  

2-phenoxyethanol is also being used (0.3 ppm)

Clove oil could be used

Thanks to everyone!

https://www.extension.org/mediawiki/files/7/72/AnestheticsinAquaculture.pdf

Water with Carbon dioxide, hypotermia 

Thank you Dr. Tomas Liptak

Polish preparation "Propiscin" shows good results.

https://www.google.com.ua/url?sa=t&source=web&rct=j&ei=xrcyVZ6GBuO_ygPagoH4Cw&url=http://www.infish.com.pl/wydawnictwo/Archives/Fasc/work_pdf/Vol20Fasc3/Vol20-Fasc3-%2520w04.pdf&ved=0CB0QFjAA&usg=AFQjCNE8nREoC_15wrU4X6j035Mg8zAiTA&sig2=64c6Hq03aLaCkV3KANH8cQ

For me clove oil or eugenol are better and safer, you can get the one for use in human (dentist supplies). The dosis has to be adjusted for your species and procedure. I have used that in cultured Sparus aurata for surgery (5 minutes procedure) and before euthanasia (1 ml/L or 4 ml/L respectively, and during 2 - 10 minutes depending the individual reactions) and with wild rocky reef juveniles A. troschelii (50 ul/L, during 30 - 60 seconds) then to clean water for surgery (3 minutes procedure) and fotographic record (2 minutes). They fully wake up after 10 minutes, then I fed them and check to see if they behave normally.

In anycase, ice-cold water or ice are now banned for animal welfare reasons, because it is an stressor itself and fish still feel pain. Tricaine mesylate (Tricaine methanesulfonate, TMS, MS-222) is also recommended, still there are questions about its effect as anesthesia or if it is only triggering a muscle weakness that leads to immobility.

Dear Zlatko,

MS-222 and extract of clove flower are used for fish species in Iran. The second one is more cheaper than the first one.

MS-222 has historically been the only drug approved for anesthetizing fish in the US.  I think they may have just approved a chemical based on eugenol as well.  Due to the long withdrawl period (~30 days) before fish can safely be eaten our IACUC has approved the use of clove oil as noted by Patricia above.  Clove oil is my preference as fish do not seem to struggle in it as they do in MS-222.  Recently several of the electrofishing manufacturers have also developed anesthesia devices using electricity to immobilize fish (see Smith-Root or Halltech web sites).  Finally, some folks use CO2.  What is best will depend on your application.  In the field in headwater streams clove oil is often most practical as we need to carry everything in with us.  If fish won't potentially be eaten then MS-222 is probably most commonly used.  In the field where consumption cannot be guaranteed against then electricity or clove oil are probably most common.

Clove oil and MS 222 give best results as we use both while clove oil being cheaper is preferred most.

eugenol or MS-222 may be the best choice, the former is more cheaper.

We normally use Clove oil.  It is easily available in most of the medical stores and highly purified one as it is used for human. We use during transportation of fish in open containers at a very low dose to avoid injuries and also used during induced breeding operation. 

For sedation, I used 150 ppm to 200 ppm of 2-phenoxyethanol.