From Creatures Wikia
The Silky-tailed Fish is a genetically modified version of goldfish. It was found on the shee spaceship and released into Albian waters by some ettins. They tend to stay aloof from their cousins, and spend their time doing what fish do - swimming around, bumping into walls (which makes a loud noise), avoiding air. Be careful you don't take your silky tailed fish out of water for too long, or it'll die. These fish also provide much entertainment and nutrition for aquatic norns.