04-24-2010, 09:48 AM
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Goldfish are members of the carassius auratus family, related to carp, and they typically like their water a little harder and with a higher pH than do most characins. Furthermore, goldfish appreciate much cooler temperatures than do the tropicals, so you would probably have to raise the temperature in the aquarium to a level that would be uncomfortable for the goldfish to keep any tropicals. Complicating this is the fact that goldfish are extremely messy fish, and in tanks, they tend to foul the water quite quickly, so fishes that are sensitive to excess waste build-up in the water probably aren't a good choice. Also, as a side note, I've seen larger goldfish swallow, even if only inadvertently, smaller fish such as neons and cardinals.
As for what companion fish I would recommend, other cool-water cyprinids would work just fine, those being: just about any danio, saving, maybe the giant, and some barbs, though some must be kept in large numbers to prevent semi-aggressive behavior.
In my experience, I've found that goldfish tend to be kept well with other carp, but not much else. Furthermore, the recommended amount of water per fish is relatively high, so not many can be healthily kept in small aquariums. That's usually fine with goldfish enthusiasts, though.