Neon and Cardinal Tetras are similar in appearance and are often confused with each other. However, there is one very easily identifiable difference. In the Cardinal Tetra, the red stripe on the lower half of the body extends the full length of the fish from the eye area to the tail. In the Neon Tetra, the red stripe only begins at mid-body, roughly below the dorsal fin, and extends to the tail.
Neon Tetras have been in the aquarium trade the longest, and are usually the least expensive of the two species.
They are also a bit smaller than Cardinal Tetras, and rarely reach an adult size of more than one inch. They do best in soft acidic water with a pH of 6.0 to 6.5 and a hardness level of 5 to 10 dGH. Neons are schooling fish, and should always be kept in groups of five or more.
Cardinal Tetras have surpassed Neons and are highly in demand in the aquarium trade. As a result, they are often priced a little more highly than their smaller and less brilliant cousins. Although they prefer soft acidic water as the Neons do, they are more demanding, preferring a pH below 6 and a hardness level below 5 dGH. Adult Cardinals will reach a length of nearly two inches. Like Neons they are best kept in schools of five or more.
Both the Cardinal and Neon Tetras are very sensitive to overall water quality, pH and hardness. For that reason, they should not be introduced to a newly set up aquarium, where changes in water parameters are inherent during the break-in period.
To ensure success, wait until the aquarium has been well established and the proper water chemistry is in place before investing in these attractive but sensitive fish.