From the outdoor ponds and glass jars, modern aquaria have evolved into a wide range of specialized systems. Individual aquaria 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 main way to classify the different aquariums is by their salinity:
Freshwater aquaria are the most popular kind of aquarium due to their lower cost and ease of maintenance.
Marine aquaria generally require more complex equipment to set up and maintain than freshwater aquaria. Along with fish species, marine aquaria frequently feature a diverse range of invertebrates.
Brackish water aquaria combine elements of both marine and freshwater fishkeeping. Fish kept in brackish water aquaria generally come from habitats with varying salinity, such as mangroves and estuaries.

Certain subtypes of aquaria also exist within these types:
Reef aquarium is a type of marine aquarium that houses coral. Since corals are quite sensitive, this kind of aquarium is considered to one of the hardest to maintain. This aquariums are usually very large - to provide a stable environment for the corals and other creatures.

Freshwater aquaria also has it's subtypes:
Coldwater aquarium, which may include fish such as the goldfish is a fairly simple-to-maintain type of aquarium. The fish are usually strong, and do not require much handling beside a water change once in a couple of weeks.

However, many aquarists maintain a tropical aquarium as these fish tend to be more colorful.
Chiclid aquarium is a type of tropical aquarium that habitat fish of the big chiclid family. These aquarium usually decorated with rocks and strong plants (or plastic ones).

A Biotope aquarium tries to mimic some natural habitat of a specific fish. The fish, plants, water chemistry, and furnishings in a biotope tank are all similar to those that can be found in a specific natural setting.

Discus fish biotope

The Planted aquarium is usually all about the plants. The goal and the challenge here is growing the plants - fish are usually not the main concern.