Pictured here is a side by side comparison of the two fish. The Cardinal is on the top, the Neon on the bottom.
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 higher 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.