There is no doubt that fish feel something akin to pain, this has been proven, and for this reason it is considered unacceptable to simply leave them to die if they are judged to be terminally ill or injured. Sadly a great many small fish when sick will die, it is more humane to assess them and if an experienced eye judges that there is no chance of recovery then the only option would be to euthanase them.
Over the years various methods have been used, many involving major trauma, and the use of an overdose of anaesthetics tended to be restricted to fish vets who routinely have these treatments available to them. Since Vetark launched Aqua-Sed, it has made euthanasia a viable option for all fish keepers and owners, using a product that has been used since the 1940's for anaesthesia and euthanasia.
The process is simple; confine the fish in a relatively small volume of water in a plastic bag and add Aqua-Sed at a dose of 4 pumps per litre. The fish will become quickly anaesthetised to begin with, and then the fish should be left for a period to become sufficiently deeply anaestheised for brain function to cease.
It is possible to anaesthetise the fish with a lower dose first, in a larger volume of water if this is preferred. Many species of fish will react to the presence of an anaesthetic by increased activity, this then makes them take the drug in more quickly. That increase in activity is the same whether being anesthetised or euthanased so we recommend going straight to the euthanasia dose if that is the intention. After this the water can be drained and the dead fish refrigerated until it can be disposed of. Disposal from trade premises requires an appropriate collection, but in the home burial or the domestic waste is permitted.