Euthanasia is a very difficult topic, and one which will no doubt affect every fishkeeper at some point during the years spent caring for their fish.For loach-keepers in particular, this may be a very difficult decision due to the longevity of some species and our subsequent attachment to our bottom-dwelling friends.
You have tried everything to nurse your fish back to health but to no avail -- it is dying and in distress.
When choosing an anesthetic a number of considerations are important such as its efficacy, cost, availability, ease of use and side effects on fish, humans and the environment (Marking and Meyer, 1985).
Overdosing of an anesthetic or retaining the fish in an anesthetic bath for too long leads to the fading of ventilation, hypoxia, and finally, respiratory and cardiac collapse (Tytler and Hawkins, 1981).
One of the ironies of the fishkeeping hobby is that despite the care frequently lavished on our pets, relatively little advice exists about the best ways to kill fish painlessly should that be necessary.
Many of the traditional methods such as the use of ice or Alka-Seltzer are notably absent from the list of euthanasia techniques available to scientists (Borski and Hodson, 2003; Brown, 2005), and in some cases are strongly criticised by experienced aquarists and vets as being inhumane (Burgess et al, 1998). Euthanasia is a way to relieving the suffering where the fish is so grievously injured or so seriously ill that treatment will not help and recovery is unlikely.
You can go this route or you can choose to speed things along for the sake of your pet.