Never heard of it. There’s no mention of this myth on Snopes either.
I found something reasonably similar in a German version of those “useless fact” lists.
Of course, it wouldn’t have to be human blood if someone wanted to bust the myth. Isn’t beef blood, pork blood, or most other sorts of mammalian blood similar enough to human blood, for purposes of sustaining goldfish respiration?
10 points awarded to Fiver for leaving the box.
And I’ll bet you didn’t know this either:
Thanks Google translation!
You know, when I looked that up I was thinking it was going to mean “fertilizer” or something. Bwa, ha, ha!