Rivulus agilae

Adults typically reach about 4 cm in length. Rivulus agilae are characterized by an elongated, slender body shape, with the dorsal and anal fins positioned closer to the caudal peduncle. The coloration and pattern on the body of this fish vary greatly. There are many geographical forms, as well as color variations even within a single population. Males display a rich variety of colors. Females of all varieties have a gray-brown body coloration and semi-transparent fins, which sometimes show a pattern characteristic of a specific geographical form. Populations inhabiting French Guiana often have a dark red body coloration, with a bright black stripe running along the lower edge of the tail, transitioning to yellow at the top. Further west, in neighboring Suriname, the fish exhibit more bluish hues; the yellow stripe on the lower part of the tail is often more saturated, and the black edging is less noticeable. Compared to their French counterparts, the body shape of these individuals is less elongated. In the western regions of Guyana, Rivulus agilae displays a bluer body coloration, and the characteristic black and yellow edges of the caudal fin are often absent or weakly expressed.

The species inhabits north-eastern South America: French Guiana, Guyana and Suriname. In their natural habitat, these fish can be found in small, sunlit, shallow streams with slow currents and in swamps. These bodies of water are typically located in savannas, near the coast. The characteristic brownish tint of the water is due to the high concentration of tannins released during the decomposition of surrounding vegetation.

Among males of this species, there is competition for group leadership and female attention. In small aquariums, such rivalry can become excessively aggressive. Therefore, for keeping in a small aquarium, it is recommended to form a group of one male and several females. Females, unlike males, exhibit peaceful behavior. Rivulus agilae can coexist with other fish species. In the wild, it is often found together with related Rivulus species. However, the behavior of males should be considered, as they can compete even with representatives of other fish species.

Keeping this fish is considered relatively easy. For a group of 4-5 individuals, the optimal aquarium volume starts from 40 L. For decoration, it is recommended to use a soft sand substrate, over which a layer of twigs and leaves is placed; natural driftwood is also used. Detailed information on choosing aquarium leaves can be found in the relevant article. Rooted aquatic plants are placed along the aquarium walls. A small amount of floating plants is permissible, provided they do not cover the entire water surface. Unlike most Rivulus species, this species is not prone to jumping out of the water, so an aquarium lid is not mandatory. The fish prefer a rather narrow water temperature range — 22-25°C. Hardness parameters should be within 5-12 dGH, and acidity — 6.0-7.0.

In nature, this species is carnivorous, feeding primarily on insect larvae and small invertebrates. In aquarium conditions, the diet should consist mainly of high-quality live or frozen foods, such as Artemia, Daphnia, bloodworms, or glassworms. Dry foods are accepted, but to maintain health and vibrant coloration, it is recommended to prioritize a variety of protein products. Feeding should be done in small portions to avoid water pollution from food remnants.

If the fish are kept in a community tank, a separate tank is recommended for successful breeding. Any container from 10 L, equipped with a simple air-driven filter, will suffice for this purpose. Low-growing thickets of fine-leaved plants, aquatic mosses, and ferns should be used as a substrate, where the fish will lay their eggs. One male and several females are placed in the spawning tank. Typically, a protein-rich diet effectively stimulates spawning. Females lay their eggs in a layer of aquatic plants. A female can lay up to 20 eggs per day. The eggs are 1.5 mm in size and amber in color. The incubation period usually lasts about 4 weeks at a water temperature of 22°C. Newly hatched fry can feed on Artemia nauplii or other micro-foods.

The species has moderate disease resistance when water parameters are maintained, but it is sensitive to the accumulation of organic compounds and sudden changes in hydrochemical indicators. The main threat comes from bacterial skin and fin infections, which arise from poor filtration or overcrowding. Prevention includes regular partial water changes, maintaining a stable temperature regime, and using quality food. If signs of apathy, scale damage, or unnatural fish behavior appear, it is necessary to check the level of nitrogenous compounds in the aquarium.

When selecting individuals, attention should be paid to the activity of the fish, the cleanliness of their skin, and the absence of fin damage. Healthy fish show curiosity, have bright, species-appropriate coloration, and a robust physique without signs of emaciation. Avoid purchasing fish that appear sluggish, stay at the water surface without other reasons, or have visible whitish spots on their bodies. Given the territorial nature of males, it is recommended to plan the group composition in advance to minimize stress and conflicts when settling them into a new aquarium.