Originally Posted by Bombalurina
That's so cool, that they stay that small! I agree with KoiMaiden - having some goldfish selectively bred to stay small entering the hobby would be great, and would help those fish whose owners insist on keeping them in smaller tanks (not aimed at the OP, I think 15 gallons is fine if they stay that small :). )
I think the big problem is that people who are just entering the hobby aren't the type to invest the level of money to make breeding them to be smaller worthwhile. People pay huge amounts for large ranchu, but not for goldfish for "My First Goldfish" kits. And, well, it is great when someone learns more and improves conditions for their fish... But I don't think the majority of people who pick up a goldfish do.
It is also easier for breeders to raise more quickly growing goldfish outdoors - they tend to deal well with temperature changes and also less likely to be stolen by birds. Smaller goldfish don't have a good time making it through their first winter, so need an indoor shelter. (Which is where one mine was sucked into the filter.)
In my opinion, fancy goldfish aren't really all that hardy to begin with... And bred to be smaller makes them even less so. There are also a lot of things that have to be considered when keeping them in a smaller tank - this type totally sucks at swimming so they never leave the feeding corner, but a more active body type would be cramped at this size. They are also very dirty, even when tiny, so tons of filtration is a must. All in all, goldfish really aren't a good first fish.