Hm i would actually say that people's estimate's for minimum goldfish tank side are even a little on the low side- non-fancy/slim-type goldfish like comets and commons can grow to 15inches+, i wouldn't start out with anything smaller than a 100gallon tank. With fancy goldfish, they vary a great deal in max size. Aat the smaller end there are bubble eye goldfish which only grow to 5inches, and at the larger end there are veiltail and fantail goldfish which can grow to 10inches+ . Personally, if you just had a pair of bubble eye goldfish i would advise setting up a 15gallon tank, but if you went for a larger variety of fancy goldfish i would advise starting out with at least a 35-40gallon tank for a pair (its best not to kept goldfish on their own as they do enjoy the company of their own kind). But a lot obviously depends a lot on the tanks measurements as well- some tanks are longer and wider than others, and these are the best for goldfish.
There isn't really such thing as a small low maintanence goldfish tank- all goldfish produce a lot of waste and so need very well maintained and properly filtered tanks, goldfish also live a very long time (20-30years or more if well looked after) and so are quite a commitment as far as fish go. But if you're really into these fish (they really are great fish BTW
), then they are well worth to effort and proper care etc- goldfish are actually surprisingly intelligent fish (as are most fish in the carp family) and are very friendly and inquisitive fish
.
With the heater thing, here's my take on this: oxygen dissolves less in water the warmer its temperature is. Goldfish need lots of oxygen to thrive, so the main drawback of keeping the tank temp warm is that you will need to aerate it a lot more for the goldfish to continue thriving- but as long as the temp isn't really warm, then the goldfish should do fine. Fancy goldfish do best in sub-tropical temps (18-23 degree's C), while non-fancy goldfish thrive best in coldwater temps (18degree's C or colder)- the main reason for this is simply that fancy goldfish were always bred to be indoor fish, so have adjusted over their centuries of breeding to more indoor temperatures. But non-fancy goldfish were only kept indoors quite recently and have always been primarily pond fish, and don't do as well as the fancy varieties in warm temps because of this.
Quick changes in temperature can be more harmful/stressful to fish than really high or really low sustained temperatures- the benefit of heating a tank is simply that it will reduce quick fluctuations in temp- if the temp of the tank gets to warm the heater will simply turn itself off, but it will prevent the tank temp falling too low. As long as you place your goldfish tank in a good place in your house though, the temp shouldn't fluctuate much (particularly if the tank is rather large), so heating isn't really necessary for goldfish tanks, since fancy goldfish do tend to do very well at average room temperature and non-fancy goldfish tend to be more than hardy enough to get through any fluctuations in temperature (as long as such changes in temp are not really extreme, like the vast changes in temp seen in a glass conservatory room or greenhouse during night and day etc). In any case, it would be an unwise idea to stick a goldfish tank in front of a window due to the algae problems that many goldfish tanks are very prone too in such situations.
Regardless of whether you decide to heat your goldfish tank or not, its always important to make sure the tank is properly filtered to help maintain lots of movement on the waters surface (to help increase oxygen content in the water) and to deal with the large strain goldfish place on the bioload of a tank from all the waste they produce etc.
