Not really sure what to say.. other then throwing it in a freezer isn't humanely putting any animal down. I have a spiney eel missing his tail in a similar wound ( but it was caused by the filter impeller not tankmates) That is doing well.. but he's alss more stremlined and lost less.. mass? If I had a standard trop lose that much mass.. I'de prolly euthanize it if I wasn't going to be willing to make consensions for keeping it alive long term.
i say keep him and do everything possible to see if he can make a comeback. it would be awesome if he did. or take him back to petsmart and say "yeah the blond girl said his tail would grow back" and then try to return him, just to see what they say! i dont think he will make it but you never know!
I would have put it out of its misory the second i found it in there with a third of its body missing. If it was just ripped up i wouldnt. But that tore apart it would be the humane thing to do. Plus leaving a fish in a tank " letting in die in peice " with other fish in there is terrible for the water paramiters. A gourami will allways get ripped apart by cichlids. At least c.a cichlids.
The fish is still alive because all of it's organs are in front of the missing parts. I would agree with the others above regarding putting the fish to sleep in the freezer.
Ever see a fish's head still alive after being cut off. That's basically what you've got there. It's probably going to die soon. If you freezer him you can use it for food. Otherwise you can use clove oil and just put a teaspoon or two in the 10g tank he's in and it will be quick.
Good luck.
I had a clown loach that got half of his body eaten (tail to fin) by another fish. Im thinking the flowerhorn. Anyways. I put him in a community tank and he gre everything back. So maybe give him some time and space and he'll be ok.
Good luck to you