Slender Rasbora

Adults of this fish typically reach a length of about 5 cm. The body is slender, with large, pointed fins. Its appearance is notable for large eyes, which seem disproportionately large relative to the body. They are adapted for foraging in low light conditions and low water transparency. A wide black horizontal stripe runs along the body from head to tail. This stripe visually divides the body into two parts: a gray back and a silvery belly. Sexual dimorphism is not pronounced and is mainly manifested in size: females are generally slightly larger and have a rounder belly than males.

Its homeland is Southeast Asia, specifically the Malay Peninsula, as well as the large islands of Borneo and Sumatra from the Sunda Islands group. These fish prefer to inhabit swamps and associated rivers, where the water has a dark brown tint due to the high content of tannins. Typically, such bodies of water are located under a dense canopy of tropical forest and are abundant in plant debris: fallen leaves, branches, and tree trunks.

This peaceful, schooling fish will be a great addition to other non-aggressive species of comparable size. The Slender Rasbora looks particularly harmonious in an aquarium with fish also native to Southeast Asia. It is recommended to keep a school of 8–10 individuals, as the more fish in the group, the more comfortable they feel.

For a small school of these fish, an aquarium of at least 60 liters is considered optimal. When decorating, it is desirable to create subdued lighting, use dark substrate, and place many shade-loving aquatic plants and various snags. Despite this, the fish can adapt to other conditions and even exist in “bare” aquariums, where they are often sold in pet stores. Water parameters should be brought as close as possible to natural conditions, not only in terms of hydrochemical composition but also in the presence of tannins, which give the water a characteristic brown tint. For this, tree leaves, such as oak, or exotic leaves and Indian almond bark, are submerged in the aquarium. Additional information is available in the article “Which tree leaves can be used in an aquarium.” Regular aquarium maintenance includes weekly partial water changes with fresh water, monitoring its parameters, and maintaining cleanliness from organic waste.

Being an omnivorous species, the fish readily accepts most common aquarium foods. The recommended daily diet may include dry flakes or granules, supplemented with live or frozen brine shrimp, bloodworms, and daphnia. It is also acceptable to feed the fish exclusively high-quality dry foods from reputable manufacturers.

Under suitable conditions, spawning will occur with enviable regularity. As with most small cyprinids, the eggs are scattered in the water column, and the parents do not care for their offspring. In a community tank, the survival rate of fry, even with a large number of eggs, will be minimal. The young become easy prey for adult fish and suffer from a lack of food, as it is extremely difficult to provide them with constant food in a large body of water. If the goal is to save the entire brood, a separate tank with identical water parameters will be required, to which the eggs will be transferred. This spawning aquarium is equipped with a simple air-driven sponge filter and a heater. No additional lighting is required — ambient room light is sufficient. For decoration, artificial plants or mosses and ferns that can grow in dim light can be used. The incubation period lasts about 24 hours, and 3–4 days after hatching, the fry begin to move freely in search of food. Specialized microfoods or Artemia nauplii are used as food.

Cottelata brittani is a hardy and undemanding fish. When suitable keeping conditions are met, health problems usually do not arise. Diseases can appear in cases of injury, contact with already sick fish, or a significant deterioration of habitat parameters (for example, in a dirty aquarium or with poor nutrition). More detailed information on symptoms and treatment methods can be found in the section "Diseases of aquarium fish."

When choosing Slender Rasbora, preference should be given to individuals with clean, transparent fins and uniform coloration without spots or film. It is important to assess the activity of the fish: healthy specimens should stay in a school and actively move in the water column, rather than hiding in corners or hovering near the surface. Avoid buying fish with signs of emaciation, sunken bellies, or damaged scales.

• Inspect the aquarium in the store for dead or sick individuals, as this may indicate the poor condition of the entire batch.
• Make sure the fish show a natural reaction to external stimuli.
• Pay attention to the absence of signs of stress or rapid breathing, which often accompany improper keeping or transportation.