I setup two quarantine tanks, the first with some 2" koi, comets, ryukin, and orandas, the second with some very fancy, slow-moving goldfish.
Both tanks seemed to be doing fine, colorful and active. The fancy goldfish were much less active, often sitting still, but I assumed that was because they had huge fins that inhibited swimming. If you measured the surface area of the fins, they probably had four times as much fin as a common goldfish.
Then there was a power outage and the second tank's pumps didn't resume its operation after the power had been restored, so in the morning, I found half of the fancy goldfish passed away. To rescue the remaining, I put them in with the first quarantine tank. Since they'd already almost reached a month in quarantine, I thought it was safe.
All of the fish in the first tank began attacking the most fancy of the fancy goldfish. They didn't appear to be eating the goldfish, but maybe just biting the scales and pushing the fancy goldfish around the tank. The fancy goldfish, having poor mobility, just bobbed around in the tank, while others pecked at it, until I managed to rescue and isolate it.
Is that a sign the fancy goldfish have parasites?