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.
About the second or third day after the young have left the eggs, they become strong enough to swim freely about in the water, and can then be transferred to the nursery, temporarily established in some part of the storage pond, which is generally not in use at this season. Here they are carefully guarded until they become about half an inch in length. For this purpose the storage pond is filled with water to a height of six inches at the deepest part; all living creatures, especially insects and their larvae, are to be removed with the aid of a fine dip-net. No plants should be placed in the nursery, as it is important that it always be under the control of the eye. Each morning the young are carefully inspected, and any enemies that may happen to have made their appearance must instantly be removed and killed. During the remainder of the time, a cover is kept over the frame in order to prevent the dragon-flies from depositing their eggs in the water, as these, when hatched, are very destructive to the young fish.
As the different lots of eggs are hatched in the jars, and the young become strong enough to swim about, they are placed in the nursery until a division of four by four (4 by 4) feet contains from four hundred (400) to five hundred. (500) fish. The next division is then prepared and stocked in the manner described, and so on until all the spawn has been so treated.
At the expiration of about a week from the time the first young were placed in the nursery, the strongest are taken out in the following manner: A candy jar is filled half-full of water from the nursery, and set right in the center of the bed and resting upon the bottom of it. The largest fish are now slowly and gently caught with a small dip-net, one at a time, and immediately in the same careful manner placed in the jar, counting them as they are transferred. It is not advisable to place more than fifty of these little fish in the jar while removing them, neither should they remain in it longer than is necessary to transfer them into the rearing ponds, the whole time consumed in the operation should not exceed a few minutes.
The morning is the proper time of day to perform this operation, as the temperature of the water in the various ponds is at that time most uniform. If the manipulation is done at any other period of the day, it becomes necessary to gradually equalize the temperature, as a difference of a few degrees only would prove fatal to the tender young if suddenly removed from one water to another.