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The Neon Tetra is a brightly colored small fish. They rarely exceed four cm in length, averaging one and a quarter to one and a half inches in length, with males being slightly smaller than females. Females also have a rounded belly, while males are described as being more 'straight'. They are named 'neon' because the blue green line that runs from their eye to their tail reflects in a way similar to neon. The female's line will have a noticeable kink in it. The function of this line is to allow other Neon Tetras to see each other in dim or dark waters. The effect of this line in an aquarium is nothing less than beautiful, particularly when they are schooling. The body of the Neon is brightly colored, with a silver fronted lower body. Their fins are typically clear, though the base of the caudal fin is a brilliant red.

Black Sharks are large fish that average 20 to 24 inches in length, though individuals as large as 2 feet have been documented. Male and females are not visually distinct, and this is one reason why captive breeding is difficult.

At maturity, the Platy will reach a size of two to three inches. They have stocky, fat, stubby bodies with a rounded caudal fin. Their bodies are usually a mixture of red, yellow, gold, black and blue. Younger Platys are usually brighter.

These are tall fish which give the appearance of a Red-bellied Piranha (Pygocentrus nattereri but they tend to be somewhat plumper despite the fact that the two are often confused. The most significant morphological difference between the two is the the level of protrusion of the jaw (much smaller in pacus than in piranhas). This helps to illustrate the largely vegetarian nature of the pacu and the largely carnivorous nature of the piranha. They are either grey or silvery fish with red bellies and the pacu has red lower body fins as well. The tambaqui also grows to a size which Pygo. nattereri can never hope to achieve. The genus to which the tambaqui formerly belonged, Colossoma was, at the time, the only genus of fish in which true molars are present. All genera which sprang from Colossoma are now among that still-somewhat-small assortment. Though they aren't to be feared as much as piranhas, tambaquis have formidable teeth which will be used on careless keepers that anger or scare them. The name Serrasalmidae means "Salmons with a Saw" (David Schleser, "Piranhas: A Complete Owners' Manual"), and this accurately describes the dentition of the Pacus.