Aquarium (á-kwā' rĭ-um), a tank for keeping living animals and plants for study and amusement. A proper proportion of plants and animals keeps the water pure, or it may be renewed. There are two kinds, fresh water and marine aquariums. The fresh water ones are more easily kept, as the animals are hardier. A good form is a square tank, about 12 inches deep, with
plate glass sides, and metal, slate or marble bottom and a metal frame. The botttom should be covered an inch deep or more with sand and pebbles scattered over it; and the tank filled with fresh river or spring water. The use of rock work adds greatly to the beauty. Among plants, water thyme, crowfoot, milfoil and starwort, are useful, because they produce a great deal of oxygen. Interesting animals for this purpose are the stickleback, goldfish, tench, gudgeon, perch, minnow and Prussian carp; while mussels and snails are good as purifiers. A saltwater aquarium needs more careful attention, but is built in much the same way. Another form, with three of the sides closed and with an inclined plane for a floor, to allow the more torpid animals easily to reach the surface, has been found very successful. Green dulse or seaweed is a good sea-plant to use, and of animals, shrimps, snails, barnacles, minnows and sticklebacks. Large aquariums have been built in many cities. One of the largest is the English one at Sydenham near London.