But since this is a digestive or nutritive process, it follows that aquatic plants may derive much or all of their food from the water itself, or the carbon in it, in the same manner as the so-called air-plant, which grows without soil, does from the air. It is true, at any rate, that, in the fresh-water aquarium, the river and brook plants need no soil but pebbles; and that the marine plants have no proper root, but are attached by a sort of sucker or foot-stalk to stones and masses of rock. It is very easy to see, then, how the aquarium may be made entirely self-supporting; and that, excepting for the larger carnivorous fish, who exhaust in a longer or shorter period the minute creatures on which they live, no external food is required.
A very simple experiment will prove the theory and practicability of the aquarium. In a glass jar of moderate size was placed a piece of Ulva latissima, or Sea-Lettuce, a broad-leaved, green, aquatic plant, and a small fish. The mouth was closed by a ground glass stopper. The jar was exposed to the light daily; the water was never changed; nor was the glass stopper removed, excepting to feed the fish, once or twice a week, with small fragments of meat. At the end of eight months both remained flourishing: the fish was lively and active; and the plant had more than half filled the bottle with fresh green leaves.
Any vessel that will hold water can, of course, be readily converted into an aquarium. But as we desire a clear view of the contents at all times, glass is the best material. And since glass globes refract the light irregularly and magnify and distort whatever is within them, we shall find an advantage in having the sides of the aquarium parallel and the form rectangular. As the weight of the aquarium, when filled with water, is enormous,—far more than we should at first imagine,—it follows that it must be capable of resisting pressure both from above and from within. The floor and stand, the frame and joints must be strong and compact, and the walls of plate or thick crown glass. The bottom should be of slate; and if it is designed to attach arches of rock-work inside to the ends, they, too, must be of slate, as cement will not stick to glass. The frame should be iron, zinc, or well-turned wood; the joints closed with white-lead putty; the front and back of glass. There is one objection to having the side which faces the light of transparent glass, and that is that it transmits too much glare of sunlight for the health of the animals. In Nature's aquarium the light enters only from above; and the fish and delicate creatures have always, even then, the shady fronds of aquatic plants or the shelter of the rocks,—as well as the power of seeking greater depths of water, where the light is less,—to protect themselves from too intense a sunshine. It is, therefore, sometimes advisable to have the window side of the aquarium made of glass stained of a green color. It is desirable that all aquarial tanks should have a movable glass cover to protect them from dust, impure gases, and smoke.
When we speak of an aquarium, we mean a vessel holding from eight to thirty gallons of water. Mr. Gosse describes his larger tank as being two feet long by eighteen inches wide and eighteen inches deep, and holding some twenty gallons. Smaller and very pretty tanks may be made fifteen inches long by twelve inches wide and twelve deep. Great varieties in form and elegance may be adapted to various situations.
There are two kinds of aquaria, the fresh- and the salt-water: the one fitted for the plants and animals of ponds and rivers; the other for the less known tenants of the sea. They are best described as the River and the Marine Aquarium, and they differ somewhat from each other. We shall speak first of the fresh-water aquarium.
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.