I do beg to differ. I have just done a search of the www using Google, for the exact phrase "aquarium amount water change recommendations", and I have not found a site that recommends a complete water change. Most people do not keep labyrinth fish in an unaerated tank, let along one without substrate.
I know of one person, who keeps her goldfish in a typical goldfish bowl, without substrate or heater, but she's the only one, in my 20+ years of keeping fish. When a tank is set up, one fish is generally added to produce a viable "good" bacteria colony. If proper water changes are done, at least 25% each week, by using a syphon to sift the substrate, then the amount of harmful bacteria is reduced to a tolerable level. Should water changes not occur on a routine schedule, then a maximum 75% water change, to reduce the amount of stress on the fish.
If you're specifically talking about a goldfish tank of mini proportions (less than 5 gallons), then yes, a complete, 100%, water change can be done, and probably should be done. Goldfish are more apt to handle the temperature change of a complete water change than any other type of fish.
I do think, in this instance, that Beaglemom is correct regarding water changes.
I'm firm - but fair. Mind the rules and enjoy your stay.
According to the Humane Society of the United States:
There are an estimated 3-4 million dogs and cats euthanized each year in the US alone! PLEASE - spay and/or neuter your pets!