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.
When fish are kept in tanks as merchandise by dealers, the location of such receptacle should be well lighted, airy, and not permitted to freeze. As was stated in another chapter, it is the oxygen contained in the water that the fish breathes, and it is therefore necessary to consider what means can be employed to keep up a continuous supply of it. One way, the most in use, although the worst for the retail customers, is to keep a stream of water constantly flowing through the tank. Now, this is wrong. Goldfish are intended to live in standing water, and should not be made accustomed to the contrary, as the reversing again of the character of the water often proves fatal to them.
The proper way to keep fish is in pure standing water, to which the necessary oxygen is supplied by the action of aquatic plants-these every dealer in fancy fish is compelled to keep on hand, if he understands the principles upon which the aquarium is managed, and if he wants to make the handling of fish a financial success.
Aquatic plants, when in a healthy condition, exposed to the light, consume the carbon in the carbonic acid gas which is produced by the fish as refuse matter, and give off the oxygen, which in turn is appropriated by the fish. This answers the question as to how many fish can be kept in a certain tank, for it is easy to understand that a locality favorable for the growth of plants will produce the greatest amount of oxygen in the water. Large fish consume more oxygen than small ones, so the proportion of fish to the tank must not be greater than the supply of oxygen the tank can produce. Besides all this, a light location is more beneficial to the color of the fish, and also affords a better control of the contents. Such a store-tank may consist of a large aquarium with glass sides, or it may be a wooden trough thirty (30) inches in width, ten (10) or twelve (12) inches in depth, and of any convenient length. The frame to form the sides and ends is made of one and one-half (1 1/2) inch stuff, the bottom being formed of flooring boards, as they are fitted with tongue and groove. To make a tight job, pieces of flag leaves, such as are used by coopers, are laid upon the edge of the frame, and the strips of flooring board nailed down securely, one at a time. The groove in each piece is thickly painted with pure white lead ground in oil, the tongue of the next then being tightly fitted into it, and so on, piece after piece, until the bottom has been completed. The best way, probably, is to nail the flooring crosswise upon the frame, as that makes the trough very strong and capable of carrying a considerable weight of water, the smooth side of the boards is of course turned towards the inside of the tank.
Such a tank, however, when in operation should not contain a greater depth than six (6) inches of water, rather less probably, both for the convenience of catching the fish and the better admission of light. The trough must be kept clean, every now and then removing all the contents and thoroughly sponging the interior.
This may appear to contradict the author's method of managing an aquarium; it may be said in explanation that dealers' tanks are, as a rule, overstocked, and therefore require a somewhat different treatment; they are, in comparison with a regular aquarium, the same as a hotel is, compared with a private residence.
While in the hands of the dealer the fish should receive a limited but regular supply of food, and should disease make its appearance, the sick fish are at once taken out and put by themselves.