I can't verify it but I thought I read somewhere in aquaculture literature the goldfish are notorious for bringing in unwanted disease and parasites.

Here's an interesting little read on goldfish and Koi:

http://www.aces.edu/dept/fisheries/aquaculture/docs/5864154-7201fs.pdf

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.