The Blue Cardinal Rasbora, scientifically known as Rasbora rutteni, is a captivating and relatively lesser-known species of freshwater fish native to the peat swamp forests and blackwater rivers of Southeast Asia, particularly Borneo and Sumatra. This species belongs to the Cyprinidae family and is prized in the aquarium hobby for its stunning coloration and peaceful demeanor. The Blue Cardinal Rasbora exhibits a slender body with a striking iridescent blue line that runs laterally from the head to the tail, complemented by a vibrant red stripe below, creating a visually appealing contrast.

Due to its native habitat's soft, acidic water conditions, the Blue Cardinal Rasbora thrives in aquarium setups that replicate these conditions. A well-planted tank with dark substrate not only enhances the fish's vibrant colors but also mimics the shaded environments of peat swamps. The addition of driftwood and leaf litter can further simulate the natural blackwater conditions by releasing tannins into the water, softening it and lowering the pH.

The diet of Rasbora rutteni is omnivorous, with a preference for small invertebrates and zooplankton in their natural habitat. In captivity, they should be provided with a varied diet that includes high-quality flake foods, micro pellets, and live or frozen foods such as brine shrimp, daphnia, and micro worms. Offering a mix of dietary items ensures they receive all the necessary nutrients for optimal health and coloration.

Breeding the Blue Cardinal Rasbora can be challenging but is certainly achievable with the right conditions. They are egg scatterers, releasing their eggs among fine-leaved plants or moss where they receive no parental care. To encourage breeding, it may be beneficial to set up a dedicated breeding tank with optimal water conditions and plenty of plant cover. The fry are small and should be fed infusoria or other fine foods until they are large enough to accept larger fare.

Caring for Rasbora rutteni involves maintaining clean water with regular changes and effective filtration to keep the water parameters stable. Their peaceful nature makes them suitable for community tanks with other similarly sized and tempered species. However, their small size and somewhat timid nature require careful selection of tank mates to avoid competition and ensure an stress-free environment.

In summary, the Blue Cardinal Rasbora, Rasbora rutteni, offers aquarists the opportunity to keep a small, colorful, and peaceful species that adds both beauty and intrigue to the freshwater aquarium. While their care requires attention to their specific dietary, environmental, and social needs, the rewards of keeping Blue Cardinal Rasboras include the enjoyment of their striking appearance and the challenge of replicating a slice of Southeast Asia's unique blackwater ecosystems in the home aquarium.