Sorry for your loss. :(
To chime in on the euthanasia issue, though--it is a very personal decision and it's really not something that anyone online can advise you on since we are not there and we cannot see when you're seeing. However, generally, I think it's best to euthanize when the fish cannot eat/has no will to eat, has difficulty breathing, and difficulty swimming. These three things seriously affect their quality of life, particularly eating and breathing. There are some curable conditions that cause loss of appetite, mainly internal parasites, but if the fish continues to show no will to eat after treatment, it's best to put them down. Conversely, if the fish continues to eat as they are deteriorating, I believe they are still showing you their will to live, so in that case, I would hold off.
As for the best euthanasia techniques--the only two I would use are the clove oil/eugenol or finquel method, or blunt force trauma. Blunt force trauma (i.e. crushing the fish in one quick blow with a blunt object such as a brick) can be quite traumatic for the owner, but it is a very quick and merciful death. It can be a viable option for people who cannot find finquel or clove oil/eugenol. Finquel is an anesthetic designed for use in fish and can be ordered here: http://www.nationalfishpharm.com/
The only local place I've been able to find clove oil/eugenol is GNC: http://www.gnc.com/product/index.jsp...74024&kwCatId=