Aquarium classifications

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

A temperate marine aquarium

More expensive and complex equipment is required to set up and maintain a marine aquaria. Marine aquaria frequently feature a diverse range of invertebrates in addition to species of fish.^[44][58] Brackish water aquaria combine elements of both marine and freshwater fishkeeping.^[58] Fish kept in brackish water aquaria generally come from habitats with varying salinity, such as mangroves and estuaries. Subtypes exist within these types, such as the reef aquarium, a typically smaller marine aquarium that houses coral.^[58]
Another classification is by temperature range. Many aquarists choose a tropical aquarium because tropical fish tend to be more colorful.^[58] However, the coldwater aquarium is also popular, which is mainly restricted to goldfish, but can include fish from temperate areas worldwide and native fish keeping.^[58]

A saltwater aquarium

Aquaria may be grouped by their species selection. The community tank is the most common today, where several non-aggressive species live peacefully. In these aquaria, the fish, invertebrates, and plants probably do not originate from the same geographic region, but tolerate similar water conditions. Aggressive tanks, in contrast, house a limited number of species that can be aggressive toward other fish, or are able to withstand aggression well. Most marine tanks and tanks housing Cichlids have to take the aggressiveness of the desired species into account when stocking. Specimen tanks usually only house one fish species, along with plants, perhaps found in the fishes' natural environment and decorations simulating a natural ecosystem. This type is useful for fish that cannot coexist with other fish, such as the electric eel, as an extreme example. Some tanks of this sort are used simply to house adults for breeding.
Ecotype, ecotope, or biotope aquaria is another type based on species selection. In it, an aquarist attempts to simulate a specific natural ecosystem, assembling fish, invertebrate species, plants, decorations and water conditions all found in that ecosystem. These biotope aquaria are the most sophisticated hobby aquaria; public aquaria use this approach whenever possible. This approach best simulates the experience of observing in the wild. It typically serves as the healthiest possible artificial environment for the tank's occupants.