Posted by Lynn Zheng on March 19, 2008

Q: I was just wondering... why gold fish live better in cool water than warm water? Is it because there are more oxygen or something?

State: Outside the U.S. and Canada

A: I'm a big goldfish fan so I'm pleased that you're asking about these great fish. Your question is a good one Lynn, but the answer I can give is only a rather general one - I can't be too specific. Goldfish are native to central Asia (you probably already know this) where temperatures can get quite cold. Native goldfish are therefore adapted for living in cooler waters. Most of the tropical fish we enjoy in home aquariums are native to the warmer areas of the world and they're not adapted to cooler waters - they'll die, or at least not thrive if too cool and this is why we need to keep their tanks on the warm side. So I think the answer simply comes down to what temperatures these fish are adapted for rather than the oxygen content of the water. Actually goldfish are especially well-adapted for living in poorly oxygenated waters so this is definitely not the reason even though cooler water does indeed hold more oxygen than warmer waters.

You probably also know that there is a great deal of variation in goldfish - some are much like the native fish while others are very, very different. Well, the more highly "bred' goldfish (the ones with fantails, bulging eyes, or other unusual features) tend to be much more delicate and are not able to tolerate waters as cool as the native fish can. In other words, if you have goldfish that are close to the native stock, these can live very comfortably in a pool outside even in the dead of winter. But, if you have highly bred goldfish, these probably would suffer if they get too cold.

Hope this helps. Thanks for asking.

John Wiessinger ( March 20, 2008)