Aquariums 

Aquariums are generally called a tank if square or rectangular, or a bowl if circular, such as a goldfish bowl. A home aquarium is a clear-sided container typically constructed of glass or high-strength plastic, in which water-dwelling plants and fish are kept for public display. The bright colours and constant movement of the fish is very enjoyable to watch. There are many varieties of fish  to choose from and  ponds or tanks are both  great  habitats for them.

Aquarium keeping is a popular hobby around the world, with about 60 million enthusiasts worldwide. It is most popular in Asia and the United States.The number of aquarists have swelled as more sophisticated systems including lighting and filtration systems were developed to keep aquarium fish healthy. 

From the outdoor garden ponds and glass jars and ceramic bowls of antiquity, modern aquaria have evolved into a wide range of specialized systems. Containers can vary in size from a small bowl large enough for a single small fish, to the huge public aquaria that can simulate entire marine ecosystems. 

The keeping of fish in confined or artificial environments is as ancient as humans eating fish. Ancient Sumerians were known to keep wild-caught fish in ponds, before preparing them for meals. Pictures of temple pools filled with Oxyrhynchus, a sacred Egyptian fish, have been found in their ancient art. In China, selective breeding of carp into today's popular koi and goldfish is believed to have begun over 2,000 years ago. 

But  a pond is one thing and a tank is another. The size of the tank limits the type of fish one can keep.So size, lighting conditions, density of floating and rooted plants, placement of bogwood, creation of caves or overhangs, type of substrate, and other factors (including an aquarium's positioning within a room) can all affect the behavior and survival of tank inhabitants. 

Aquariums contain fresh water or salt water and brackish water, at tropical or cold water temperatures depending on the type of fish and other inhabitants that can survive and thrive in the aquarium. Saltwater is typically alkaline, while the pH of fresh water varies more. A careful aquarist spends a lot of time  maintaining a tank ecology that mimics its inhabitants' natural habitat. 

Freshwater aquaria remain the most popular due to their lower cost and easier maintenance, but saltwater aquaria have gained in popularity with dedicated enthusiasts prepared for the challenge. Freshwater can be relatively easily created from tap water by removing the chlorine, while saltwater requires the addition of a sometimes complex mix of salts and minerals. 

Most fish tolerate only small changes in water temperature so water temperature is important. Tropical fish are relatively easily maintained around 25C in a warm climate. Coldwater fishes may be harder to maintain within our comfortable room temperature. Water temperature is regulated with a combined thermometer/heater or cooler unit.

A filtration system is needed to keep the nitrogen level acceptable and keep the water sediment free. The inflow and outflow of water movement through the filter can be controlled through the use of powerheads and may also create a careful amount of internal water flow which may be good for some breeds of fish. 

The types of fish kept together is also important as is the ratio of the quantity and size of fish to the available quantity of water. Overcrowding fish is not good for them and putting two males of some breeds (like bettas) together is not wise, nor is putting a large fish-eating fish among small fry.