sometimes i have had fish that have been badly battered or suffering due to illness. recently i lost my largest tinfoil to (i think) a swim bladder infection. he started to swim upside down the he just rested on the substrate upside down, pleco started to try and eat his eyes. i took him out and put him in a poly box with meth blue and lots of air but it did not help him and he died himself, but i have had other fish i had to euthanise.