The emanations from paint and putty are often fatal in new tanks. Several weeks' exposure to water, air, and sunlight is necessary to season the new-made aquarium. Of equal consequence is it that the water be absolutely pure; and if brought from the sea, care must be exercised about the vessel containing it. Salt acts upon the glazing of earthen ware of some kinds. Stone or glass jars are safest. New oak casks are fatal from the tannin which soaks out; fir casks are safe and good. So delicate and sensitive are the minute creatures which people the sea, that they have been found dead on opening a cask in which a new oak bung was the only source of poison. And no wonder; for a very slight proportion of tannic acid in the water corrugates and stiffens the thin, smooth skin of the anemone, like the tanning of leather. A certain natural density of the sea-water must also be preserved, ranging between no wider limits than 1026 and 1028. And in the open tank evaporation is constantly deranging this, and must be met by a supply from without. As the pure water alone evaporates, and the salts and earthy or mineral constituents are left behind, two things result: the water remaining becomes constantly more dense; and this can be remedied only by pure fresh water poured in to restore the equilibrium. Hence the marine aquarium must be replenished with _fresh water_, until the proper specific gravity, as indicated by the hydrometer, is restored. The aquarium may be found some morning with a deep and permanent green stain discoloring the water. This unsightly appearance is owing to the simultaneous development of the spores of multitudes of minute Algae and Confervae, and can be obviated by passing the water through a charcoal filter. When any of the fishes give signs of sickness or suffocation, by coming to the surface and gulping air, they may be revived by having the water aerated by pouring it out repeatedly from a little elevation, or by a syringe. The fishes are sometimes distressed, also, when the room gets too warm for them. A temperature of 60° is about what they require. And they will stand cold, many of them, even to being frozen with the water into ice, and afterwards revive. The degree of light should be carefully regulated by a stained glass side, or a shade. Yet it must be borne in mind that sunlight is indispensable to the free evolution of oxygen by the plants. And when the sun is shining on the water, all its occupants appear more lively, and the fishes seem intoxicated--as they doubtless are--with oxygen. A novice is apt to overstock his aquarium. Not more than two moderate-sized fishes to a gallon of water is a safe rule. Care, too, must be taken to group together those kinds of creatures which are not natural enemies, or natural food for each other, or a sad scene of devastation and murder will ensue.