Check the facts. Look up goldfish ,
Carassius auratus, on fishbase.org. It is a site that is used and maintained by scientists, not hobbyists. This makes it one of the if not THE most reliable source of accurate information in regards to max size, natural and introduced geographic range, pH, temp, etc. Here is a link to the page on goldfish so you can see for yourself:
http://fishbase.org/Summary/speciesSummary.php?ID=271&genusname=Carassius&speciesname=auratus+auratus&AT=carassius+auratus⟨=English
I think it is a good time to get a little more specific. Not all goldfish can handle the same temp range. In general the fancier the breed a goldfish is the more sensitive they are. So a common goldfish is hardier than a fantail, which is hardier than an oranda, etc. I lump the breeds in to two major categories: the long-bodieds and the round-bodieds. The round-bodieds are all the fancies like moors, ranchus, etc. The long-bodieds are all the breeds with a natural body form regardless of finnage (so it includes comets, wakins, etc.). The long-bodieds are much hardier overall, especially when it comes to temperature tolerance. These are the types that are in ponds year round in any climate. These are the types that when released or escaped can establish naturalized populations in almost any climate. The fancies are not nearly as temperature tolerant. They cannot go as cold or as warm as long-bodieds. So although they shouldn't be in a pond year round (although some people do keep them this way without issue) they also shouldn't be up at discus temp either, at least not long term.
I have had goldfish for years in many different circumstances. Many people may not agree, and I understand, but the first time I had any breed was a bunch of sarassas in a 75 with discus. Here is a video:
[video=youtube;e7UqQJS-ZBI]http://www.youtube.com/watch?v=e7UqQJS-ZBI[/video]
My fancies (kept at room temp) did not do the same until years later.
Here is a map I made based on the FACTS found in the link to fishbase above (not regurgitated myth). The nations in blue are where goldfish are from naturally. Even without any human interference they are in the quite tropic areas of Laos, Myanmar, etc. The nations in red are all the places goldfish have established naturalized populations (introduced and now stable). As it should be obvious, obviously they are not the coldwater fish we have been led to believe. Obviously temperature is not an issue. Add to this the fact that most tropical tanks are only a few degrees above 'coldwater' room temp tanks and the idea that they cannot thrive in tropical conditions should simply crumble away to nothing.
