I know I build things for fish to live in, but why is it called an aquarium?
I  have a big Aquarium in my back yard , is a vivarium of any size having at least one transparent side in which aquatic plants or animals are kept
oh cool!  What kind of fish can go in them?
As a Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aquatic reptiles such as turtles
Can I just put water in there?  Or is there anything else I need to do?
water-filled tank in which fish swim about. Small aquariums are kept in the home by hobbyists. There are larger public aquariums in many cities. This kind of aquarium is a building with fish and other aquatic animals in large tanks. A large aquarium may have otters, turtles, dolphins, and other sea animals. Most aquarium tanks also have plants.
An aquarist owns fish or maintains an aquarium, typically constructed of glass or high-strength acrylic. Cuboid aquaria are also known as fish tanks or simply tanks
Aquarium
An aquarium (plural: "aquariums" or "aquaria") is a vivarium of any size having at least one transparent side in which aquatic plants or animals are kept and displayed. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aquatic reptiles such as turtles, and aquatic plants. The term "aquarium", coined by English naturalist Philip Henry Gosse, combines the Latin root "aqua", meaning water, with the suffix "-arium", meaning "a place for relating to".