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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?

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    Please say, what dimensions are this tanks? And where will the fishes go after the quarantine time? – Allerleirauh Mar 24 at 6:43
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    Both quarantine tanks are 20 gallons. Planned to move them all to a 20k gallon pond at the end of quarantine, this weekend. – Village Mar 24 at 13:40
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    I'm wondering if they are pecking at microscopic parasites on the scales, as I know some fish do like to clean other fish scales. – Village Mar 24 at 13:42
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    For me it sounds more, that the 20 gallon is too small and the residents defend their territory against the new goldfish. But I am no expert :) – Allerleirauh Mar 24 at 15:18
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Too long for a comment, so an answer that might help future readers.

How old are the fish? What sex? Sometimes pushing about is male fish trying to squeeze eggs out of female fish. In my experience, female goldfish get eggs round about 3 years of age. Males get breeding tubercles about 1 year. Sometimes biting and pushing just fish defending territory, showing dominance or defending their share of the food.

If parasites were present:

  1. You might see the parasites. A lot of parasites are surprisingly obvious.
  2. The fish will be itchy so they’ll flick themselves off of the gravel to try to scratch. Even if they’re slow swimmers, they’ll still try to do this.
  3. You might see the effects of parasites (look for holes in the fish, signs of poor condition).

Lastly, some fish tanks have, like, culture? (Ok, as much culture as you expect goldfish to have!). That is, the fish as a group have nap times and bed times and territories. Fancy goldfish might have an area they consider a bed and they might have an afternoon nap (all together). This can cause problems mixing groups of fish.

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