Family Characidae or American Tetras ( Characidae )
Group of Uncertain Status ( Incertae Sedis )
Genus Paracheirodon.
Large fish should not be kept with neons (which should be kept in groups of at least 5–7 specimens, preferably many more), as neons are easily perceived as prey by large fish and quickly fall victim. When purchasing, inspect the fish in the shop aquarium with great care. Fins must be clear, body skin without defects, bellies should not be hollow or pinched, and the fish must show active, natural mobility. If there are any signs of distress, white or grey patches on the skin, fading color, paleness of the red stripe, or spinal curvature in even a single fish in the tank — avoid purchasing. These are prime symptoms of the incurable disease known as “Pleistophorosis” or “Neon Tetra Disease”.
All fish infected with pleistophorosis perish, contaminating the water, plants, and tankmates. There is no cure; medications only suppress spore development without eradicating them. Consequently, all fish, plants, and mollusks from an infected aquarium will have to be humanely destroyed. The aquarium tank and all equipment must be thoroughly disinfected. Exercise caution!
— Cardinal Tetra — Paracheirodon axelrodi [Schultz, 1956]. Synonyms: Cheirodon axelrodi, Hyphessobrycon cardinalis. (English: Cardinal Tetra, Large Neon Tetra, Red Neon Tetra, Scarlet Characin). Brazil, Venezuela, Colombia, the Orinoco and Rio Negro river basins. Inhabits shallow forest streams and floodplain lakes of the upper Rio Negro and tributaries of the Orinoco. 4 cm.
The back is greyish-violet with an iridescent sheen. A shimmering green or blue horizontal stripe extends from the eye to the adipose fin, beneath which the body—with the exception of the throat and the whitish-yellow belly edge—is vibrant red. Scales are large and prominently visible over the iridescent stripe, which dims almost entirely in the dark and is replaced by a short, broad blue-violet sheen. The red tone turns light brownish.
All fins are transparent. The outer margins of the dorsal and anal fins possess a bluish-white edging. In this neon, the red stripe covers the entire lower belly. Females are larger with a broader iridescent stripe and a more convex belly profile; their anal fin is concave, whereas males have a slightly elongated, convex anal fin with an almost straight outer edge. Males display a straighter blue line, whereas in females it appears curved due to the fuller abdomen.
At 3 weeks of age, fry develop their iridescent lateral stripe and their eyes begin to glow. By 5 weeks, juveniles attain the full coloration and body shape of adult fish and can be moved to a community tank. Sex can be determined before full sexual maturity: at 4–5 months of age, this is apparent from the shape of the anal fin (noticeably concave in females, almost straight to slightly convex in males). A golden captive strain exists — P. axelrodi var. “Gold”.
A schooling, active, and completely peaceful fish. Keep in groups of no fewer than 10–15 individuals so their dazzling colors do not fade against other tank inhabitants. The larger the school, the more magnificent the visual display. They swim primarily in the lower and middle strata of the tank. They get along splendidly with various small tetras and quickly form a tight, persistent school. Do not house with large, boisterous, or aggressive fish.
The aquarium should feature dense vegetation along the back and side panels while providing ample open swimming space. Bright illumination should be diffused using floating plants to establish shaded retreats. The fish dislike intense glare, which causes them to school closely along the dimmest glass wall seeking shade.
Water must be soft, free of nitrates, and preferably peat-filtered and acidic (pH 6.0 is preferable to 7.0). They are warm-water fish requiring temperatures not lower than 24°C; the primary requirement is thermal stability: 26–30°C. To maintain optimal vitality, replace about a quarter of the tank volume bi-weekly. Readily accept live, frozen, and dry foods. Females reach maturity at 8–9 months; males around 4–5 months later. Lifespan is up to 5 years. pH 5.5–6.5; T 23–28°C; dH 2–8°.
Identification: Cardinal Tetra — Paracheirodon axelrodi.
— Green Neon Tetra, Blue Neon Tetra, False Neon Tetra — Paracheirodon simulans [Gery, 1963]. Synonyms: Cheirodon, Hyphessobrycon simulans. (English: Blue Green Neon Tetra, False Neon Tetra, Green Neon Tetra). Colombia, Venezuela, Brazil; inhabits slow-moving forest creeks and sluggish waters of the Orinoco and Rio Negro basins. 3 cm. Diminutive, delicate, and less widespread in the hobby than its congeners, it is imported and bred much less frequently.
Distinguished from its close relatives—the Neon Tetra and Cardinal Tetra—by its slender, elongated body profile. The dorsum is dark olive-green, fading to lighter shades below. A broad dark longitudinal band runs along the flank, tapering toward the caudal fin. While the famous iridescent neon stripe in P. innesi and P. axelrodi starts at the snout and terminates before the adipose fin, in P. simulans it extends all the way to the base of the caudal fin.
This stripe is an intense blue-to-turquoise hue. The lower abdomen displays a turquoise sheen, which is particularly evident in gravid females. In some individuals, the anterior rays of the dorsal and anal fins are whitish, and scales along the dorsal ridge feature grey speckles producing a subtle, pale silhouette outline.
The red-magenta stripe is short—much like that of the classic neon tetra—often subdued, and can disappear completely under suboptimal water conditions. The caudal peduncle is brick-red, sometimes extending into the caudal and anal fins. The anal fin is longer than the dorsal and bordered by a delicate uneven stripe. Females are larger and deeper-bodied than males. P. simulans is set apart from the Cardinal Tetra, which it closely resembles, by its vivid turquoise-green line running uninterrupted to the caudal fin base.
Requires delicate care and attention during acclimation. Peaceful and lively; keep in schools of at least 10 specimens to showcase their iridescent colors. They should never be housed with larger species that could prey on them. Thickly planted areas (including floating flora) along the back and sides of the aquarium are essential, balanced with open swimming space.
Rock caves, bogwood roots, and clay pots serve well as shelters. A dark substrate and dark aquarium background are strongly recommended. Fish cruise open areas in the mid to lower water columns. They favor dim, subdued lighting with shaded areas. Very sensitive to water quality: they demand soft, slightly acidic, peat-conditioned water. Efficient filtration, steady aeration, and weekly water changes are advised; peat filtration or blackwater extracts can be utilized.
General hardness should not exceed 6° dH; otherwise, skin ulcerations and mortality may occur. They accept small live and dry foods (particularly quality flake food). In nature, they feed on fallen terrestrial insects and micro-crustaceans. Prone to velvet disease (Oodinium). Sexual maturity is reached at 7–12 months. Not recommended for beginners. Lifespan is up to 5 years. pH 5.5–6.5; dH up to 6°; T 22–27°C.
Paracheirodon innesi, Neon Tetra, “Diamant”, “Gold”.
Nematobrycon lacortei, Nematobrycon palmeri, Nematobrycon amphiloxus. 
