blind goldfish

i used to have 2 one-eyed goldfishes following their being terrorised by one particular gangster goldfish. one had died and only one survived. it's been 6 months since he (or she or it... watever) had it's eye dug out. and tis morning, i discovered to my horrors that his (or her or its'...) other remaining eye had also been gorged out. and so for the most of today, i have one totally blind goldfish swimming weakly in the tank blind. i am not optimistic about it's chance of surviving past this evening. it was weak this morning, and by this afternoon, it was already swimming upside down. and looking at it, i thought it had died and wanted to remove and dispose it. but when i lifted it out, it started gasping weakly for air. i had to put it back into the tank. i am now counting down the final moments. will it be in terms of minutes? or hours? or worst still, days? i really don't know. but all i know is that he/ she/ it is suffering. and it is during times like this that i feel helpless and wonder if mercy killing is the right thing to do. then again on second thoughts, i shall leave it to die on it's own. i do not think i should be playing god. am i doing the right thing? :-(