How to euthanize your fish?

I have 6 black neon tetras who I found the other day had Ick. I immediately put the treatment in and began treating them for ich and it now appears that they are getting worse. Before there were 2-3 white spots now they have +10 spots they are swimming around and appear very stressed. I would like to know how to euthanize my fish if they don't get better. I can't do anything physical as I dont want to see their mangled bodies. I would like to take them out in the most humane way as possible