be covered with a layer of sand from one to two inches thick, upon the top of which should be placed a few pebbles, in patches. For marine aquaria, the sand must be procured from the sea-beach, or a river-side, below the tide-mark, and, for those of fresh water, from the bed of a brook or running stream, and, in either case, it must be thoroughly washed, to free it from impurities. Rock-work may be built upon the sand, to form caves and grottoes, but there should not be any hiding-places for impurities to collect in. A pleasing effect is secured by building the rock-work above the surface of the water, and making a small hollow in the top, where a fern may be planted. The rock should be taken from the sea, or the brook, according as it is wanted for the marine or fresh-water aquarium, the object being to make the artificial home of the animals as much as possible like their natural one. In constructing the rock-work, Portland cement should be used, and it should be allowed to harden before the water is poured into the tank. The water should be poured off several times, until it remains quite clean, and the greatest care should be taken to prevent the addition of impurities with any object. The plants must be stuck into the sand with a pebble tied to the roots of each to hold it in position, and they should be arranged to produce a pleasing effect, as individual taste may dictate. After the plants have taken root, the animals may be added. If both forms of life be present in proper proportions, there will be no necessity for change or disturbance of the water, but, if they be not, artificial aeration will be required. When this is needed, the
Fig. 3.
Fresh-water Aquarium.
fish will repeatedly rise to the surface and stick their noses out to catch a breath of oxygen. Aëration may be effected in lifting the water by cupsful, and allowing it to run back gradually from a point two or three feet above. This process should be continued for a half-hour or more at a time, once or twice a day. The ordinary level of the water should be indicated by a mark on the glass-front, so that loss from evaporation can be detected and supplied. Great care must be taken to secure uniformity of temperature, between 45° and 65° Fahr. For this purpose an east window should be chosen where the sun can be
