Aphyocharax rathbuni

Adults of this fish reach a length of 3 to 4 cm. Sexual dimorphism is weakly expressed, so the differences between males and females are minimal. Young fish have a silvery coloration with a reddish tint on the lower part of the caudal peduncle. Over time, their coloration acquires yellow or golden tones, and the transparent anal and dorsal fins turn red. White markings can be seen on the other fins.

Its natural habitat is located in South America, covering the basins of the Paraguay, Paraná, and Uruguay rivers. These bodies of water extend across the territories of the eponymous countries, as well as in southern Brazil and northern Argentina. It prefers to live in small rivers and streams, where it finds shelter in the shade of overhanging and aquatic vegetation.

This peaceful and active fish feels best in a school of at least 6–8 individuals. It is well compatible with other non-aggressive species of comparable size. However, it should be noted that the excessive activity of Aphyocharax rathbuni can cause discomfort for some calmer fish. Therefore, the choice of tank mates should be approached with caution.

For comfortable keeping of a school of 6–8 individuals, an aquarium of 40–50 L will be optimal. In the setup, it is recommended to use a sandy substrate and dense thickets of aquatic plants. At the same time, they should be arranged in such a way that free areas for swimming remain. The fish is not demanding regarding the hydrochemical composition of the water and is easy to maintain, easily adapting to a wide range of pH and dGH values. However, such unpretentiousness does not negate the importance of maintaining high water quality. The accumulation of organic waste, which can lead to a dangerous increase in the concentrations of nitrogen cycle products (ammonia, nitrites, nitrates), is unacceptable. The biological balance in the aquarium largely depends on the regularity of maintenance and the uninterrupted operation of the main equipment, including the filtration system.

This species is omnivorous. Aphyocharax rathbuni readily accepts various foods intended for aquarium fish. Its daily diet may include dry flakes and granules. These are recommended to be combined with live or frozen brine shrimp, bloodworms, and daphnia.

Favorable conditions for spawning are achieved in slightly acidic and soft water. The fish scatter their eggs among plants; thanks to sticky threads, the eggs attach to their surface. One female can lay up to 500 eggs. The incubation period lasts 2–3 days, and after just one week, the fry begin to swim freely. Parental instincts are not developed in this species; the fish do not care for their offspring and may eat them if given the opportunity. If breeding is planned, the eggs or hatched fry should be moved to a separate tank with identical water parameters.

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 Aphyocharax rathbuni, attention should be paid to the overall activity of the school and the cleanliness of the skin. Healthy fish should be active, without visible damage, ulcers, or film on the body. Avoid individuals with signs of emaciation, sunken bellies, or unnaturally bloated abdomens. Remember that young specimens may appear paler than adults, but their coloration should gradually develop with proper care.

Consider the structural features of males: their anal fins have special hooks that can catch on a net when caught. When purchasing, it is important to ensure that the fish have not sustained fin injuries during transportation or transfer at the store, as any damage can lead to infections or render the male unable to reproduce. It is advisable to acquire a group of 6–8 individuals, as this is a schooling species that experiences stress when kept alone.