For turtles, crawfish, and lizards a scap-net will be necessary. This may be made from stout wire, a broom-handle, and some netting. The ring may be almost any size, from six to twelve inches in diameter, the ends being sharpened with a file and turned so that they may be driven into the end of the stick, which should first be bound with wire to prevent it from splitting. The bag of mosquito-netting is made on the hoop and sewed fast, as shown at Fig. 20. When changing the water in the aquarium it is not necessary to empty it all out. A siphon made of a small rubber tube will answer very well to drain off a portion. The part of the tube in the tank should be held close to the bottom so as to suck up any dirt or sediment that may be there. Good ventilation, light, healthy aquatic plants, clean sand, and a proper proportion of fish to the volume of water are absolutely necessary to the successful aquarium. A few tadpoles and snails are advantageous in an aquarium, as they consume decaying vegetable matter and help to prevent the formation of algæ on the glass.
For the aquarium large enough to contain them, some artificial rockeries may be made from cement, gravel, and stones, as shown in the illustrations of the concrete rockeries Figs. 21 and 22. They should be made with openings beneath for the fish to swim through, and pockets should be made at the top to hold sand and the roots of aquatic plants. The rockeries should have a good flat base so as to rest securely on the bottom of the aquarium. Always have plenty of pebbles and river-sand at the bottom of the aquarium to make the fishes feel at home.
It is generally supposed that it is necessary to change the water in an aquarium at least once a day; but that is not the case. The true principle on which an aquarium should be conducted is not to change the water at all, but so to aërate and refresh the original supply as to maintain it always in a pure and perfect state. There are several means by which this may be done. The healthy growth of plants is very important, and active and brisk contact with the air of the atmosphere will greatly freshen the water. Motion in the water is absolutely necessary. In large aquaria this is obtained by an arrangement of tanks into which the water is pumped, and from which it flows rapidly, circulating through the tanks where the fish live. In its passage through the air it absorbs considerable oxygen, without which no fish can live. Fish placed in water that has been boiled die in a very few minutes.