This section is from the book "The Goldfish And Its Systematic Culture With A View To Profit", by Hugo Mulertt. Also available from Amazon: The goldfish and its systematic culture.
The day before the fish are transplanted from the nursery into the rearing pond, the water is let into the latter, but not sooner than this. The reason for this is, that the water, if allowed to stand longer, would produce an over abundance of food, and with it, of course, a host of dangerous insects. If the young fish were put in the pond, while in this condition, the insects would at once give chase and devour them.
The little food these tiny fish require, during their first days of existence, is found in sufficient quantities in the new water. By the time their enemies make their appearance, especially those of the insect kind, the fish have grown too strong for them, and are very well able to look out for themselves.
In transferring the young from the jar, to the rearing pond, it is much better to sink the jar and allow the fish to make their escape at will. This precaution enables them to gradually become accustomed to the change, thus avoiding a shock by the sudden emptying of the jar.
In this manner the young are "planted" in the rearing ponds, at the rate of 250 to each pond, of 8x20 feet in size. Here they remain until they have acquired their coloring, and have grown large enough to be saleable.
This may be variously from six weeks to four months, according to circumstances, and the care expended upon them.
Though the instructions detailed above were given with reference to cultivation in the open air, yet by following them the same thing can be done in the parlor, but on a small scale, and with limited resources. The writer has often delighted his customers by furnishing them with sprigs of water plants, upon which eggs were adherent. These they hatched in a glass tumbler, placed upon the window sill, gradually increasing the size of the vessel, and consequently the quantity of water as the young grew up, until they were finally ready for permanent residence in the aquarium. Such specimens are generally looked upon with great pride by their owners because they are home-bred and grew up in the midst of the household, where all could watch them and learn a bit of natural history without an effort. The spawn for this purpose can frequently be obtained from any well managed aquarium, if the habits of the goldfish are understood.