A freshwater aquarium is a receptacle that holds one or more freshwater aquatic organisms for decorative, pet-keeping, or research purposes. Modern aquariums are most often made from transparent glass or acrylic glass. Typical inhabitants include fish, plants, amphibians, and invertebrates, such as snails and crustaceans. This term refers to all aquariums whose salinity, measured on a hydrometer calibrated to 3.98 °C, results in a water density of 1,000 g/cm³.

Freshwater fish may be either coldwater or tropical species. Although freshwater aquariums can be set up as community tanks, coldwater and tropical fish are generally not mixed due to incompatibilities in temperature requirements. Coldwater aquariums house goldfish and other species that do not require a heating apparatus. Warmer temperatures would actually increase their metabolism and shorten their lifespan. For a tropical fish tank, maintaining a warm environmental temperature ranging between 24 and 27 °C enables different tropical fish to thrive at different temperatures.

Aquarium substrate usually consists of sand or gravel. Aquarium substrate is important for the fish's health and replicating their natural environment. Aquarium decoration can consist of live or plastic plants, driftwood, rocks, and a variety of commercially made plastic sculptures. Though rocks and plants have more practical purposes for the health of the fish and the water quality. The smallest aquariums are fish bowls, but these are not recommended for most fish as they are generally too small, tend to stunt fish growth, and may lead to eventual death.

The earliest known aquariums were artificial fish ponds constructed by the ancient Sumerians over 4500 years ago. The ancient Assyrians, Egyptians, and Romans also kept fish in ponds for food and entertainment purposes. The ancient Chinese were the first culture to breed fish with any degree of success. They raised carp for food around 2000 BC, and developed ornamental goldfish by selective breeding. Goldfish were introduced to Europe during the 18th century.

In the later 18th century, widespread public interest in the study of nature was awakening, and fish were kept in glass jars, porcelain containers, wooden tubs, and small artificial ponds. During this time zoologist and botanist, Johann Matthaeus Bechstein, kept many fish and amphibians and laid down the foundation for aquarium and terrarium science. The concepts of the proper aquarium and terrarium were developed later by Nathaniel Bagshaw Ward in 1829.

During the 19th century the idea of the "balanced aquarium" was developed. This approach was an attempt to mimic a balanced ecosystem in nature. According to this method, fish waste could be consumed by plants, and plants along with the air surface of the water could supply oxygen for the fish. In 1869, the first tropical fish (the Paradise Fish) was imported from Asia. In these days, tropical tanks were kept warm by an open flame. Because early filters were noisy and expensive, fishkeeping was a hobby reserved for wealthy, scientifically inclined individuals.

In 1878, Rear-Admiral Daniel Ammon brought the first tropical fish from the Far East to the United States which led to a decline in the popularity of goldfish. In the early 20th century, aeration, as well as particulate and charcoal filtration was introduced. The undergravel filter was introduced in the 1950s. By this time, the old idea of the balanced aquarium was viewed as unattainable and unnecessary by many people in the aquarium hobby, but it made a comeback at the end of the 20th century with the rising popularity of the planted tank.

Today fishkeeping has become a popular hobby that almost anyone can do. Aquarium fish are both wild-caught and bred throughout the world. Captive-bred species are inexpensive and widely available, and are less likely to be infected with diseases or parasites. Unfortunately, successive generations of inbred fish frequently have less color and sport smaller fins than their wild counterparts.