Rivulus caudomarginatus

Adults reach approximately 6 cm in length. Their body is elongated with short fins. The coloration is predominantly grey with a noticeable blue sheen, and the belly has a silvery hue. The body pattern consists of numerous dark dashes and two distinct spots: one located immediately behind the head, the other at the base of the caudal fin. Males are easily distinguished from females by their brighter and more contrasting pattern, as well as their colored fins; in females, the fins and tail remain semi-transparent.

The species inhabits South America, primarily the coastal regions of Brazil (states of Rio de Janeiro, São Paulo, Santa Catarina). Its natural environment includes brackish and freshwater biotopes, including stagnant bodies of water, swamps, and mangrove areas. The fish prefer areas with dense vegetation, dark substrate, and calm water flow.

This fish species is considered peaceful, though somewhat timid. They prefer to stay in groups with their conspecifics. Keeping them with other species is permissible if the fish are comparable in size, especially if they also inhabit brackish waters.

For a group of 3–4 Kryptolebias caudomarginatus, the optimal aquarium volume starts from 60–80 liters. The aquarium setup should include an abundance of shelters created by dense plant thickets. A dark substrate is preferred, supplemented with a layer of leaves and snags. Although Kryptolebias caudomarginatus can survive in fresh water, they will feel most comfortable in a brackish environment where the salt concentration is 10–15 grams per 1 liter. It is important to maintain a medium to high water hardness level, and the pH should be above 7.0.

The daily diet of these fish should be based on live, frozen, or fresh foods with a high protein content. Excellent choices include Artemia, Daphnia, and bloodworms.

The fish reach sexual maturity at 5–7 months of age. They spawn regularly, laying eggs in soft, silty substrate. Notably, the size of the eggs, about 2 mm, is quite large for such miniature fish. Despite this, a single clutch can contain several dozen eggs. The incubation period usually lasts 2–3 weeks. Information sources: FishBase, ItrainsFishes

Fish are sensitive to water quality and sudden changes in its chemical parameters. If keeping conditions are violated, especially with insufficient filtration or accumulation of organic matter, the risk of bacterial infections and fungal skin lesions increases. Prevention includes maintaining stable environmental parameters, regular water changes, and observing aquarium hygiene.

When choosing specimens, attention should be paid to the condition of the fins and skin, which should be free of white film, ulcers, or signs of damage. A healthy fish shows activity, has clear eyes, and a normal appetite. It is recommended to avoid purchasing fish that appear lethargic, stay at the surface, or show signs of emaciation, as the species is considered difficult to adapt to aquarium conditions.