MESSRS. EDITORS:—There is one feature whieh no writer on the aquarium has yet noticed: when a tank is properly stocked, the water soon gets crowded with animalculie, which swarm among the plants, and occupy the sides of the glass in countless numbers, made visible only by the aid of the microscope. These facts are in accordance with certain laws of natnre, and the presence of vegetable and animal life always develops them. But observe the utility of these animal-culm ; they contribute to the sustenance of the other living creatures by snpplying them with food. The researches of chemistry have proved that these minute organizations respire in much the same way as plants, while animals generally absorb oxygen, and perish if the introduction of that gas is suspended. These minute organisms absorb carbonic acid gas, and give out oxygen in abundance. My experience convinces me that a tank which has been fitted np for some months, will sustain a much greater amonnt of animal life than one of the same dimensions bnt recently stocked. Beginners should distinctly rQmember the leading principles of the aquarinm, and then success in maintaining one may, without much difficulty, be achieved. If the tank have not a distinctly self-supporting character, such as will preserve its strength without alteration of any kind, it may be concluded that there has been unskillful management in its stock. H. D. Butler. [The above communication will, no doubt; be interesting to those of our readers who have begun to stock aquaria; but we think the writer's remarks apply more especially to salt water tanks.Eds.