For those into the chopping the heads off, I've tried this once and it's not very easy to do!
You place the fish on a board and if it flops around, there's gonna be some sumarai accuracy in your cut else you might cut it at the belly - not a good sight with all the guts spilling out and the head twitching before it goes...not a good sight especially since it's your pet - speaking from experience there.
If the fish is eating, that's a good sign, but I think for these type of disease, you need to starve it - after a few days you feed it again (trying soaking food in garlic - it may help too). Also from the pix, looks like it's got dropsy and some type of infection on it's back. Multiple ailments usually = death.
You place the fish on a board and if it flops around, there's gonna be some sumarai accuracy in your cut else you might cut it at the belly - not a good sight with all the guts spilling out and the head twitching before it goes...not a good sight especially since it's your pet - speaking from experience there.

If the fish is eating, that's a good sign, but I think for these type of disease, you need to starve it - after a few days you feed it again (trying soaking food in garlic - it may help too). Also from the pix, looks like it's got dropsy and some type of infection on it's back. Multiple ailments usually = death.