The great secret in establishing a self-supporting aquarium is to establish a natural balance of water, fish, plants, and light, so that none of these agents are wanting in quantity. For instance, a strong light is required to cause a healthy development of the plant life, but not direct sunlight, or the plants will be forced too rapidly, and death will soon follow. Again, direct sunlight will increase the temperature of the water to such an extent that many of the fish will die. If the animal life is in excess of the plant life and the water contained in the aquarium, the animals will perish for want of sufficient air. Again, if the aquarium is overstocked with plants, so that they are crowded so closely that the light fails to reach some of them, decomposition will take place, and everything will become a decaying mass. In fact, it is only by beginning on a very modest scale, with a very few and small fish at first, and by gradually increasing the number, that a beginner can expect to succeed. Over-stocking with animal life and overfeeding are the two greatest temptations that beset the path to success for the aquariumist; but patience, perseverance, and critical observation will eventually lead to success. The greatest care must be taken, and all shells, rock-work, sand, and plants must be washed over several times, so that no injurious substances may be introduced. Ulva, or sea-lettuce, is to be found in abundance in all our small bays and inlets at low tide. For the aquarium, those specimens which are thick in texture, and of a dark green color, only are fit for manufacturing air. Never be tempted to make use of the light green and thin specimens, as they are not sufficiently matured, and will soon decay if placed in the tank. [Illustration: SCALLOPS.] Scallops when young have a curious way of changing their location by means of opening their shells and then closing them with great force, which sends them off at an angle, and so they go dancing along the bottom till they reach a spot that suits them. This shell-fish forms a beautiful addition to an aquarium. [Illustration: HERMIT-CRAB WITH SHELL.] The silver-shrimp, with figured back (all other varieties must be avoided), I have always considered as constituting a Board of Health in an aquarium; for no sooner does the water become unhealthy than these transparent and grasshopper-like creatures will make desperate attempts to jump out of the tank. These shrimps, and the little hermit-crab, and the buccinum (a small black sea-snail) are Nature's house-cleaners. They are always on the look-out for decaying animal or vegetable matter, which, if not in too large quantities, they speedily devour.