Epalzeorhynchos frenatum (Green Fringe Lark / Rainbow Shark)

02 August 2026

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The Green Fringe Lark, widely known as the Rainbow Shark, is a striking freshwater cyprinid native to Southeast Asia. Famous for its vibrant red fins contrasting against a dark body, this semi-aggressive bottom-dweller brings dynamic activity to large aquariums, though its territorial nature requires careful tank mate selection.

Epalzeorhynchos frenatum (Green Fringe Lark / Rainbow Shark)

Overview

The Green Fringe Lark, more commonly known in the aquarium trade as the Rainbow Shark or Ruby Shark, is a highly popular freshwater fish. Despite the "shark" moniker, it is actually a species of cyprinid. Known for its striking contrast of a dark body and fiery red fins, it adds a brilliant splash of color and high energy to the lower levels of a suitably sized aquarium.

Distribution and Habitat

This species is native to the river basins of Southeast Asia, primarily found in Thailand, including the Mekong, Chao Phraya, and Mae Klong basins. They inhabit the sandy and rocky bottoms of rivers and streams where they graze on algae and biofilm. During the rainy season, they are known to migrate into flooded areas.

Image of green fringe lark epalzeorhynchos frenatum

Description of Green Fringe Lark

Body shape:

They possess an elongated, torpedo-like body that gives them a distinctly shark-like silhouette. The ventral profile is relatively flat, which is ideal for a bottom-dwelling lifestyle.

Color:

The main body exhibits a dark grey to olive-green or brownish coloration. Depending on the lighting and the fish's mood, the body can appear almost black or take on a subtle greenish hue.

Barbels:

As seen closely in their facial structure, they have two pairs of short, sensitive barbels protruding from the corners of their mouth, which help them locate food in the substrate.

Snout:

The snout is pointed with a slightly downturned mouth, perfectly adapted for grazing and scraping algae off flat surfaces.

Eyes:

They have relatively large, alert eyes with a dark pupil surrounded by a lighter, sometimes golden ring.

Fins:

All of the fins—dorsal, pectoral, pelvic, and anal—are a vibrant, striking red or deep orange, providing a beautiful contrast to the muted body color.

The Tail:

The caudal fin (tail) is deeply forked and shares the same vivid red/orange coloration as the other fins. A distinct dark, almost black patch is often visible right at the base of the caudal peduncle.

Temper

The Green Fringe Lark is considered semi-aggressive. While they are generally indifferent to fish that occupy the upper water column, they are highly territorial when it comes to the bottom of the tank.

Sociality

This is a solitary species that does not play well with its own kind or other shark-like fish (such as Red Tail Sharks or Flying Foxes). Keeping more than one in a standard aquarium almost always results in severe aggression and dominance disputes. They should be kept as the sole bottom-dwelling "shark" in the tank.

Activity

They are incredibly active and industrious fish. You will constantly see them darting around the lower levels of the tank, patrolling their territory, and grazing on decorations, plants, and the substrate.

Keeping and Caring for Green Fringe Lark

Tank size:

Due to their adult size of up to 15 cm and their highly territorial nature, a minimum tank size of 55 gallons (approx. 200 liters) is required. A larger footprint is always better to provide ample grazing space.

Water Parameters:

Temperature

They thrive in tropical temperatures ranging from 22-28°C (72-82°F).

pH

The ideal pH should be kept slightly acidic to neutral, between 6.5 and 7.5.

Hardness

Water hardness should be maintained between 5 and 15 dGH.

Ammonia, Nitrite, Nitrate

Ammonia and Nitrite must always be at 0 ppm. Nitrates should be kept as low as possible, ideally below 20 ppm, through regular water changes.

Lighting

Moderate lighting is preferred. If the lighting is too intense, ensure there is plenty of shaded cover provided by plants or hardscape.

Water flow

As a riverine species, they appreciate moderate to moderately strong water flow to simulate their natural environment.

Decorations and Plants

The tank must have plenty of hiding spots. Use smooth rocks, driftwood, and PVC pipes to create caves. Dense planting is highly recommended to break the line of sight and help reduce territorial aggression towards other tank mates.

Diet

They are omnivorous scavengers. In the wild, they consume algae, insect larvae, and zooplankton. In the aquarium, they will readily accept high-quality sinking pellets, algae wafers, and occasional live or frozen foods like bloodworms and brine shrimp.

Some Feeding Tips

Ensure that food actually reaches the bottom of the tank before mid-water fish consume it all. Feeding them right before turning the tank lights off can help ensure they get their share without competition.

Breeding

Breeding the Green Fringe Lark in a home aquarium is extremely difficult and rarely achieved due to their intolerance of one another. In commercial settings, breeding is typically induced using hormone injections in large outdoor ponds.

Conclusion

The Green Fringe Lark (Rainbow Shark) is a visually stunning and fascinating fish that makes a great centerpiece for a large, semi-aggressive community aquarium. As long as their territorial requirements are respected and they are provided with plenty of space and hiding spots, they will be a hardy and entertaining addition to your aquatic setup.

Fish info

Care level: Moderate
Temperament: Semi-aggressive
Adult size: 15 cm
Life expectancy: 5-8 years
Fish type: Cyprinid
Temperature : 22-28°C
pH: 6.5-7.5
Color Form: Black, Green, Grey, Orange, Red

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