The tank being prepared, and well-seasoned, by being kept several weeks alternately full and empty, and exposed to the sun and air, so that all paint, oil, varnish, tannin, etc., may be wholly removed, the next thing is to arrange the bottom and to plant it. Some rough fragments of rock, free from iron or other metals that stain the water, may be built into an arch with cement, or piled up in any shape to suit the fancy. The bottom should be composed entirely of shingle or small pebbles, well washed. Common silver sand, washed until the water can be poured through it quite clear, is also suitable. Mould, or soil adapted to ordinary vegetation, is not necessary to the aquatic plants, and is, moreover, worse than useless; since it necessitates the frequent changing of the water for some time, in order to get rid of the soluble vegetable matter, and promotes the growth of Confervae, and other low forms of vegetation, which are obnoxious. Aquatic plants of all kinds have been found to root freely and flourish in pebbles alone, if their roots be covered. The plants should be carefully cleared of all dead parts; the roots attached to a small stone, or laid on the bottom and covered with a layer of pebbles and sand. The bottom being planted, the water may be introduced through a watering-pot, or poured against the side of the tank, so as to avoid any violent agitation of the bottom. The water should be pure and bright. River-water is best; spring-water will do, but must be softened by the plants for some days before the fishes are put in. Sunshine is good for the tank at all seasons of the year. The fresh- requires more than the salt-water aquarium. The amount of oxygen given off by the plants, and hence their growth and the sprightliness of the fishes, are very much increased while the sun is shining on them. In selecting plants for the aquarium some regard is to be paid to the amount of oxygen they will evolve, and to their hardiness, as well as to their beauty. When it is desired to introduce the fishes without waiting long for the plants to get settled and to have given off a good supply of oxygen, there is no plant more useful than the _Callitricke_, or Brook Star-wort. It is necessary to get a good supply, and pick off the green heads, with four or six inches only of stem; wash them clean, and throw them into the tank, without planting. They spread over the surface, forming a rich green ceiling, grow freely, and last for months. They are continually throwing out new roots and shoots, and create abundance of oxygen. Whenever desired, they can be got rid of by simply lifting them out.