The quickest way to euthanise a small fish is to wrap it in a couple of paper towels and whack it on the head with something hard and solid (hammer). If you can't do that, put it in a small container of aquarium water and pop it in the freezer. Leave until frozen solid then bury the ice block in the garden.
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You can use salt or Praziquantel to treat fish for gill flukes. Praziquantel also treats tapeworm.
Use 4 heaped tablespoons of salt for every 20 litres of tank water. Keep salt levels up for 4 weeks. Then dilute it out.
or
Treat with Praziquantel once a week for 4 weeks. Do a 75% water change and gravel clean the substrate 24-48 hours after each treatment.