*WHEN NEMO ISN'T GETTING BETTER*
There is plenty of death and disappointment in this hobby. After so many times you, as an aquarist, will be able to judge when your fish isn't going to get any better.
So what can you do other than watch your fish struggle?
One way is to try clove oil. Clove oil can be found at pharmacies and sometimes at grocery stores. It is also known as "eugenol". It acts as a sedative. Get a liter of tank water and about 20 drops of oil. Your fish will be put into a comatose state. Depending on the size of your fish, it can take anywhere from 10 seconds to 10 minutes. When the gills are no longer moving, it is over.
But most of us do not do this. Freezing is viewed as an acceptable practice.
It is as easy as putting the fish in a plastic bag and placing it in the freezer. The cold will deaden the nerves so that pain is not felt. It is a cheap method, but all materials are usually on hand.