We have a young Diamond Tetra who has suddenly gone wacky. One minute he was swimming around without a care in the world. Now he is flying about the tank in an uncontrolled spiral-like pattern, much like a balloon that was blown up and then let go without tying the end. It appears that he has had some sort of fish stroke, as the right side of his body is mostly paralyzed, and is body is kind of curved around to the right. Yesterday I figured he only had a few hours to live. Tonight he is still flailing about the tank. Anyone have any suggestions on how to put our little fish out of his misery?
Do you have a cat?
A couple of methods would be so quick they would be relatively painless.
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Catch it and drop it into a bowl or container of ice water. (Place a bowl of water in the freezer until it just starts to freeze, then break the surface ice.) The temperature shock should kill a tropical fish almost instantly.
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Drop it into alcohol at as high a concentration you can find. (Vodka would work.) Again, this should almost instantly paralyze and kill the fish.
I remember discussing this problem with some biologist colleagues, and that’s a tough one.
I think that Colibri gave you two very good solutions. There is another one that I can think about, which may sound like a joke but which would actually provide the desired result: use a kitchen blender.
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Catch the fish in a net and smash the net on the nearest hard surface. Seems brutal but the fish will never know.
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Decapitate the fish with a sharp knife.
Ach, so sorry, but I’m afraid that this will not work.
Solution 1: due to the light weight of the fish, and the dumping power of the net itself, you will find it difficult to smash the fish hard enough. And the little beast is quite resistant so it may take you quite a few attempts before you manage to kill your fish this way -if this ever happens.
Solution 2: the fish will fight, try to escape, slip around on its slimy skin; you will have a hard time getting a good grip, and it will take you some time before you manage to complete decapitation. If you don’t cut your fingers in the process.
Out-of-water fish handling is difficult for you and definitively stressfull for the animal. You want to avoid it as much as possible.
I can, however, suggest an efficient variant of your solution 1. Capture the fish in a small net, then use a large hammer…
maybe he’s just horny
If it’s small, I think the kitchen garbage disposer solves merciful dispatch of life and corpse disposition simultaneously. Fire up the disponser and drop it in. I’ve seen arguments on my fish hobbyist sites against icewater. Alcohol seems a bit fancy, and not necessarily instantaneous. Smashing is fine but seems a bit messy.
With the bettas I’ve had, I went with the ice water then pop the container in the freezer for awhile method. At least it was quick. I don’t have a disposer, so that option is out.
The thought of trying to decapitate a betta is hilarious. Hilariously awful, but hilarious nonetheless. They’re way too little to get a good grip on.
Do you want your last memories of your fishy friend to be of a brief, but incredibly painful death? If not, the clove oil and vodka method is relatively simple and uses readily available products. (Most grocery stores or pharmacies carry clove oil.)
Also, congratulations on wanting to treat your pet humanly.
A few quotes from that website, Bolt the Nut:
I just put my fish to sleep. I followed the directions on this site exactly as stated, and the clove oil did not put my fish to sleep. I waited and waited and still nothing, so I added a bit more, and it began to irritate them. They gasped for air at the top, and I could see the skin around their eyes was all red, which it had not been before. My fish looked worse than ever and were apparently suffering worse than ever, and I did not know what to do at this point so I had to kill them in an extremely horrible way - at least to me it was - and I don’t even want to explain it to people on this site. I needed to act fast, and your website doesn’t tell us what to do when the oil doesn’t work, and it’s doing damage. I got the exact oil you recommend and double checked it with my vet. I was completely unprepared for this situation, and I feel horrible.
I found this article when I was looking for a humane way to send a group of guppies to their watery heaven. A man gave me a tank full of fish that were just awfully twisted, deformed and sick. I suspect they had tuberculosis, but I suppose it could have been strictly inbreeding. I decided to use the clove oil method and it worked great. The fish slowly fell asleep. No spaz attacks or stress. After about 30 minutes of them being asleep, I added the vodka and their gills stopped moving. It made me feel better knowing that they went peacefully after a life of such pain. Then, I had a plecostomus that I thought had a huge patch of fungus. (It turned out to be Argulus) I used just 1 drop of clove to about 1 pint of water. So, I slowly added a few drops of the mixture. After a few minutes, he started spazing out like crazy. He was literally trying to suck up the sides to get out of the mixture. I figured he was just freaking because he didn’t know what was happening. It turns out that it actually killed him a few minutes later. I don’t know if armored fish are somehow different, but he completely spazed and died and I was so upset. I felt like I murdered one of my favorite fish. So, tonight I looked at one of my Purple Moscow Guppy babies. He had a tiny hair stuck in his mouth and he couldn’t get it out. So, I made the same mixture. 1 drop to about 1 pint. I added a few drops and he slowly fell asleep. I used tweezers and pulled out the hair, it was long and hard to get out. Then, I slowly took out the old water and added some of the tank water. I have been slowly doing this for about 45 minutes. Take out old water, add tank water. He looks dead and pale. But I still see his tiny heart beating. I have no idea how long to wait for him to wake up, but if he doesn’t wake up within the next hour, I’m going to euthanize him. I figure 2 hours in clean water and he should be moving. Or id this wrong? Be very careful when using clove oil to put fish to sleep temporarily. It hasn’t worked well for me so far!
Well, this method did not work well on our goldfish. 
I’m really upset as I was trying to find a peaceful method for him and I think I made it way worse than if I would’ve just chopped his head off. I got 100% clove oil (Humco brand), put one dose in the water, he did not fall asleep and he was really distressed by the clove oil addition. After waiting 10 minutes, he was more relaxed, but still swimming, so I added more clove oil and waited another 10. Now it seemed like he was asleep, his gills weren’t even moving, so I poured in the vodka. Then he woke up and started thrashing again. It is horrible. (He is in 16 oz of water and I added 4 oz vodka). He is about 3 inches long, so maybe I needed a higher dose of the clove oil to begin with, but still, the vodka should have done him in by now and his gills are STILL MOVING. Christ. I feel horrible. I don’t recommend this method at all. Go and get some finquel. I looked in three pet stores for it and couldn’t find it, which makes me sad that the pet stores aren’t even euthanizing their fish in a humane way.
etc etc etc
Doesn’t sound too good to me!
I am also amazed that there are people this attached to fish. I suppose there’s no logical difference between them and other pets, just never come across people who gave two hoots about fish before 
What are those? It may not work rapidly enough on cold-water fish, but I would think it should be quick enough on tropicals. But I can’t say I have tried it myself.
The fact is, nothing is going to be completely painless or non-traumatic to the fish, unless perhaps you go to the trouble of getting a fish anesthetic that veterinarians use and overdose it. So the quicker you can do the deed the better.
Drop it into a pot of boiling water. Instant death that is most likely painless.
Then add a bit of white wine, cream, tarragon…
It was time for a water change anyway so I put about a gallon of tank water into the sink with Mr. Sickfisch, turned on the disposal and pulled the plug. Probably very disorienting for the fishy, and maybe painful for a 10th of a second. But certainly better than suffocating in the sewer or carrying on like he was for the next 24-48 hours.
I really don’t like watching anything suffer, even the fishes.
Do fish get tuberculosis or was that a joke?
I don’t mind to sound callous - I understand that, from the owner’s perspective, this might matter. But is there really any need, from the fish’s perspective, to ensure a humane death? Their brains are roughly insect-sized, right? When I squash a mosquito, I don’t do it to be cruel - but I don’t particularly care whether it suffers, either.
I think the freezing solution sounds like the bet one.
Here’s [a site](a site) that discusses euthanasia of fish.
Top contenders with a humane rating of 100%:
[ul]
[li]Drop into hard alcohol like vodka.[/li][li]Drop or bathe in MS-222 (Tricaine Methanesulfonate anesthetic, but use a lethal dose)[/li][li]Electrocution (easy on the fish, not so great for you so don’t screw around)[/li][li]Drop into freezing water (for tropicals, not coldwater fish).[/li][li]Drop into boiling water (for coldwater fish, not tropicals).[/li][/ul]
I’d say the garbage disposal is probably just about as humane, from a quickest is best standpoint.
“WHOOPS! Shredded Sushi.” Is anyone else having a Zak McKracken moment here? (Electrocution was an option, too. Still better than what happens to the hamster in Maniac Mansion…)