Q. I’ve set up a 30-gallon aquarium to house some goldfish. I haven’t quite decided what I want yet, but a salesperson at my local pet store said to not mix feeder-type goldfish with fancy ones — no explanation given. Is there a reason to keep the two types separate?
A. It’s not that you can’t keep regular and fancy goldfish in the same aquarium, but there are two reasons why goldfish hobbyists do not recommend mixing feeder goldfish and show (fancy) goldfish. The first is that feeders tend to be fast, frisky swimmers. They are built for swimming.
Fancy goldfish, in contrast, tend to be slow, ungainly, fat and not well equipped to compete for fish food. Thus, if you mix them in the same aquarium, your feeders will get most of the food and grow rapidly, but your expensive fancy goldfish will barely get enough to stay alive. That’s the most important reason not to keep the two types together.
The second reason is that all goldfish are goldfish — meaning they can and will breed together regardless of their physical appearance. The feeders are close approximations of wild goldfish (see the first question in this column for a detailed explanation). The fancy goldfish are distant genetic strains.
If the two goldfish breed, the result will be a regression toward the characteristics of wild goldfish, which lack the color, form and finnage that make fancy goldfish so special and so expensive. Moreover, because the feeders are far more vigorous than fancy goldfish, there is a good chance that breeding will result in serious physical harm to the fancy goldfish. Goldfish breeding tends to be a rough and tumble affair.