why dont you just throw it as hard as you can at the wall, or put it on the table and smash it with a hammer?
Beating a fish against a wall is NOT going to kill it quickly. Fish can take an amazing ammount of physical abuse and still be alive. This is a cruel way to try to kill a fish.
The hammer would work, but be way more messy than a knife.
One thing about the knife/hammer method, it is going to take you a bit of time to get the fish where you want it, line it up, and finish it off. This time spent out of water is really stressful to fish. Just drop the fish in ice cold water with clove oil, takes about 2 seconds to get the fish from the tank to the cup and its dead.
I'm curious, why would you need to put a fish down?
I came across my issue where I had a freshwater tank with a horrible bacterial and fungal infection that would not go away. I would lose a few fish here, few fish there and tried treating the whole tank to kill it for good. Eventually, the infection became resistant to pretty much everthing, I had two extra tanks going try to find something that worked but it was a waste of effort. In the end, the fish were getting infecting and dying within hours no matter what I did. I removed all the meds from the tank, and as soon as a fish showed signs, I killed it. I think 2 or 3 fish lived through all that, sad times...
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