Exodon

These are fairly small fish: in an aquarium adults usually reach 10 cm in length, though some literature quotes up to 15 cm. In a home aquarium, their growth is usually less — up to 10 cm. Sexual dimorphism is minimal: males differ with slightly more elongated dorsal and anal fins and appear slimmer. The predominant coloration is silvery, with red fins. Characteristic features are two large dark spots: one located on the body behind the gill covers, the other at the base of the tail. The unique structure of their mouth, with sharp, protruding teeth, allows them to tear fragments from the bodies of other fish, like pincers.

These fish are widely distributed in a significant part of the Brazilian Amazon, and also found in Guyana (South America). Their usual habitats are sections of rivers flowing through savannas and other steppe landscapes, such as pastures or agricultural lands. In such biotopes, moderate currents and sandy substrates are characteristic.

Despite their modest size, these fish are extremely dangerous and bloodthirsty creatures. They should be kept exclusively in a species-only aquarium. No other fish, even large and predatory, will feel safe in their presence. Exodons can resemble a group of small aggressors who are also ready to attack each other; outwardly, this creates the impression of constant internal conflicts. The aquarist should be prepared for the appearance of outcast fish, weakened due to lack of food or injuries. If timely measures are not taken, they can quickly be killed and eaten.

The minimum recommended aquarium volume is 120 liters. The decor can be varied, but a sandy substrate and several snags are often used for decoration. However, given the behavioral characteristics of these fish, it is advisable to provide several shelters in the form of dense plant thickets, supplemented with branched snags or decorative elements such as sunken ships or castles. These places will allow fish under attack from conspecifics to find refuge and recover, which will be inevitable. Exodons produce a significant amount of waste, and uneaten meaty food scraps heavily pollute the water. Only a high-performance filtration system, combined with weekly partial water changes (15–20% of the volume) with fresh water, can cope with this. In addition to cleanliness, optimal keeping conditions include moderate current and lighting, a water temperature range of 23–28°C, and a pH from acidic to slightly acidic with any water hardness (from soft to hard).

This carnivorous species in its natural environment feeds on various insects, small crustaceans, and also small fish if they can fit into their mouth. Otherwise, Exodons will, if possible, bite off fins, tails, scales, and body parts from other fish, or from weakened or sick conspecifics. In aquarium conditions, they accept all types of meaty foods, including chopped mussels, shrimp, fish pieces, worms, and insects. Despite living in a school, during feeding, the principle of “every fish for itself” applies. Therefore, it is important to ensure that all fish receive a sufficient portion of food. Otherwise, over time, some individuals may weaken from hunger, and their fate will be sad.

Breeding these fish in a home aquarium is possible, though it presents certain difficulties. There are many nuances, making success accessible only to the most persistent or experienced aquarists. The optimal solution is to set up a separate spawning aquarium. A tank of 40 liters or more will suffice, with just a few plants. A fine-mesh net or a layer of glass marbles about 1 cm in diameter is placed on the bottom. This design protects the eggs from the parents, as they fall through the net or roll into the gaps between the marbles. The equipment set is similar to the main aquarium, but the filtration system should consist of a simple sponge filter to avoid sucking in fry. A stimulus for spawning is a gradual reduction in water hardness to 1–5 dGH and establishing a slightly acidic pH value within 6.0–6.5. After some time, some fish noticeably round out — these are females full of eggs. Future breeders (the strongest and healthiest individuals are chosen) are placed in a separate tank, prepared in advance and filled with water from the main aquarium, where they should spend about a week. It is important to regularly change the water in the spawning aquarium and remove waste products, as a weak filter will not be able to provide sufficient cleaning. Spawning can be initiated by lowering the temperature to 23°C by replacing water with cooler water during the day. After egg-laying, the fish are returned. Fry appear within 2–3 days. They should be fed microscopic food, such as Artemia nauplii and/or infusoria, then they can transition to adult Artemia and Daphnia. It is also worth noting that fry may feed on each other, but this is normal practice for them and should not cause surprise.

Paradoxical Exodons are generally resistant to diseases. The only real threat to them is poisoning from the decomposition products of organic waste, such as excrement and uneaten food particles. Thus, regular and thorough aquarium maintenance minimizes the risk of diseases.

When purchasing Exodons, it is important to acquire the entire group at once, as adding new individuals to an established school often leads to aggression and injuries. The optimal group size is at least 10–12 individuals, which helps distribute intraspecific aggression. Avoid buying single fish or too small groups, as this inevitably leads to the death of weaker individuals. When inspecting them at the store, ensure there are no damages to fins, scales, and eyes, which are common signs of skirmishes between fish. Healthy individuals should be active and show interest in food. Given their predatory nature and sharp teeth, use thick bags for transport to avoid punctures. It is advisable to prepare a quarantine tank in advance for the adaptation of new fish before introducing them into the main aquarium.