Freshwater Flounder: Species Profile & Care Guide

Freshwater flounders are as equally strange and beautiful as their marine counterparts. With their odd anatomy — flat bodies with both eyes on one side — you can easily identify these species in an aquarium.

Nonetheless, freshwater flounder species are less common and well-known than sea-thriving flatfish. They are often confused and misidentified due to the lack of information about them.

Worry not, though! As we have gone through the hassle for you. This guide will discuss the common freshwater flounder species you should know, from their appearance and needs to their behavior. Let’s start!

Freshwater Flounder Quick Facts

What Is a Freshwater Flounder?

Freshwater flounders are unique fish species adapted to live in freshwater, making them distinct from other flounder species. They belong to the Achiridae family and fall under numerous genera. Commonly, they inhabit the freshwater regions of South America, thriving in sandy or muddy substrates.

With over 200 flatfish species under the Achiridae family, confusing freshwater flounders with their marine cousins is easy. With that, here are some common freshwater flounder species you should know:

• Apionichthys nattereri: This species inhabits the Amazon River and reaches a maximum length of 23.4 cm (9.2 inches). They are characterized by a distinct morphology, including a pronounced dark coloration on most of the blind side of their head. This dark coloration is replaced in larger specimens by a gray, diffused color.
• Apionichthys finis: Although closely related to A. nattereri, this species is distinguished by its separate dorsal, caudal, and anal fins. They are also native to the freshwater environments of Peru.
• Pnictes asphyxiatus: Known for its remarkable ability to thrive in low-oxygen environments, this species occupies diverse freshwater habitats that experience varying oxygen levels, showing its adaptability.
• Catathyridium jenynsii: Recognized for its distinct body markings, this species prefers freshwater habitats. They can be found in South America, specifically in the Paraná and Uruguay River basins.
• Hypoclinemus mentalis: Also known as the Peruvian freshwater sole, these flatfish species feature a slightly elongated body shape compared to other soles and share a similar environment.
• Hogchoker (Trinectes maculatus): These flatfish are also known as hogchokers. They are adaptable to various freshwater environments and are recognized for their hardiness. Hogchokers can grow up to 20 centimeters (8 inches).
• Drab sole (Achirus achirus): Commonly known as drab sole, this flounder species inhabits the brackish water of Western Central Atlantic, from the Gulf of Paria to the mouth of the Amazon River.

These flounders are all distinguished by their unique body structure, with both eyes on one side of their body, which is an adaptation for a bottom-dwelling lifestyle.

This adaptation allows them to blend into their surroundings effectively, making them efficient ambush predators.

Their diet typically consists of small invertebrates found within their habitat, and their preference for muddy or sandy substrates provides them with excellent camouflage and protection.

Due to the variety of species, their size and specific habitat preferences can vary, but they generally share common traits suitable for life in freshwater environments.

Fun Fact: Flounders and Halibuts, despite looking similar, are not from the same species of flatfish!

Freshwater Flounder Origin and Habitat

Freshwater flounders, a group of flatfish primarily found in freshwater habitats, have a unique evolutionary story. These species are predominantly demersal, living and feeding on or near the bottom of water bodies.

These fish have adapted to life in rivers, streams, and lakes. Their origins trace back to ancestral marine species that gradually adapted to freshwater environments, a transition evidenced by their physiological traits.

Freshwater flounders are mainly found in South American freshwater systems, such as the Amazon, Orinoco, and Essequibo rivers. They thrive in muddy or sandy substrates, providing camouflage and aiding their predatory lifestyle.

Species like A. nattereri and P. asphyxiatus are native to the Amazon basin, while C. jenynsii resides in the subtropical Paraná and Uruguay River basins.

Meanwhile, hogchokers span the Amazon, Orinoco, and Essequibo River basins, illustrating their wide habitat range.

Some species, like hogchokers and drab soles, exhibit amphidromous traits, able to move between freshwater and marine environments of the Western Central Atlantic. They are also often found in brackish waters, where fresh and saltwater mixes.

Freshwater Flounder Physical Characteristics

Freshwater flounders, with their intriguing flat bodies and eyes on one side, are fascinating fish adapted to life in rivers and lakes.

Their unique body shape helps them to hide in the sandy or muddy bottoms of water bodies, making them excellent at catching prey by surprise. Depending on the species, they can grow from 3 to 24 centimeters long.

A. Nattereri

A. nattereri can grow up to 23.4 centimeters long. Their standout feature is the dark color on the blind side of their body, which interestingly turns gray in larger fish.

These flatfish also have a notable gill opening on the eyed side, resembling a small slit, and they possess a significant number of scales ranging from 92 to 114.

A. Finis

Meanwhile, A. finis are distinguished by scales covering almost their entire head, except for the nasal area and a narrow part near the cheek region in larger fish.

Unlike A. nattereri, these species lack a connecting membrane between their dorsal, anal, and caudal fins. They have two pelvic fins closely positioned together on the underside of their body.

P. Asphyxiatus

P. asphyxiatus are smaller in size, reaching only about 9.6 centimeters. These freshwater flounders have an unusual feature of lacking a gill opening on the side of the head where their eyes are located.

C. Jenynsii and H. Mentalis

On the other hand, C. jenynsii and H. mentalis are both over 20 centimeters long, distinguishing them as among the larger species within the freshwater flounder family.

Hogchokers

Hogchokers are also unique flatfish that reach about 20 centimeters. They are usually found in brackish waters where rivers meet the sea, feeding on small creatures in the mud. They have a distinctive color pattern of dark brownish-gray with narrow black lines or spots.

Drab Soles

Lastly, drab soles feature an oval body with small eyes on one side and a rounded snout. Their body is covered with rough scales and clusters of hair-like structures, known as cirri, which are more prominent in smaller fish and reduce in number as the fish grows.

These descriptions highlight the unique adaptations of each freshwater flounder species, showcasing how they have evolved to suit their specific aquatic environments.

Freshwater Flounder Temperament and Behavior

Freshwater flounders are known for their peaceful and non-aggressive nature, making them suitable for communal aquarium settings.

They are bottom-dwellers, often preferring to stay near the substrate of rivers and lakes. This behavior is part of their survival strategy, allowing them to blend into their environment and ambush prey.

In aquariums, they tend to hover near tank sides or bury themselves in the substrate. They are nocturnal and engage in feeding and other activities during this time.

A unique aspect of many flatfish, including freshwater flounders, is the metamorphosis they undergo during their juvenile stage. In this process, one eye migrates to the other side of the head, leading to their characteristic asymmetrical appearance.

This adaptation is vital for their bottom-dwelling lifestyle, allowing them to lie flat against the substrate while keeping both eyes facing upwards.

Here’s a video of how freshwater flounders behave in their natural habitat:

Freshwater Flounder Lifespan and Health

Freshwater flounders generally have a lifespan of 3 to 5 years in optimal conditions.

Their health and longevity are closely tied to their environment and diet quality. These fish are hardy but susceptible to common fish ailments, mainly if tank conditions are not ideal.

Common health issues in freshwater flounders can include malnutrition, often indicated by fin nipping in a community tank. Malnutrition can be addressed by providing a varied and nutritious diet.

Other potential health problems include bacterial, parasitic, or fungal infections, which can occur in poor tank conditions. Regular monitoring and prompt treatment of any signs of illness are important.

Freshwater Flounder Care Guide

Caring for freshwater flounders involves understanding their unique needs and behaviors. These fascinating fish, known for their flattened bodies and bottom-dwelling habits, require specific care to thrive in captivity.

Diet and Nutrition

Freshwater flounders are carnivorous, feeding primarily on small invertebrates in their natural habitat. In an aquarium setting, they should be given a balanced diet of high-quality fish pellets or flakes designed for flounders.

To engage them in their natural predatory lifestyle, I supplement the diet of my freshwater flounders with live foods like bloodworms, brine shrimp, or daphnia.

It is not only fascinating to watch them ambush their prey, but it also provides the nutrients and stimulation they need.

Feeding time for freshwater flounders should ideally align with their natural behavior, which is nocturnal. This means they are more active and more likely to feed during the evening or night.

Offering them food in the late afternoon or early evening can be a good practice.

Since these fish are bottom dwellers and more active at night, feeding them when the tank lights are dim or just after they are turned off can encourage their natural feeding habits.

Tank Setup

Replicating their natural habitat in your tank setup is key. Fine-grained marine sand is ideal for freshwater flounder tanks, as these fish often dig and bury themselves.

The tank should be decorated with non-toxic driftwood and live plants. You should also ensure a strong filtration system and maintain regular cleaning routines, including water changes and substrate cleaning.

Starting with a 10-gallon tank for juveniles is adequate, but a larger tank of at least 20 gallons is necessary​ as they grow.

Water Conditions

Caring for freshwater flounders in an aquarium requires attention to several key aspects, particularly water conditions. The water parameters should be adjusted accordingly, depending on the flounder species.

For strictly freshwater species, such as C. jenynsii and H. mentalis, the water conditions should closely mimic their natural habitats.

These environments typically include a pH level of around 6.5 to 8.0 and temperatures that range from 72°F to 76°F (22°C to 24°C).

They thrive in environments with soft to moderately hard water, with a general hardness (GH) of around 4 to 10 dGH.

In contrast, species like the hogchoker and the drab sole are amphidromous and can adapt to a range of salinity levels and water hardiness ranging from 8 to 14 dGH.

It is crucial to regularly test and maintain the water parameters, including pH, temperature, salinity, and hardness, to ensure they remain within the acceptable range for the specific species of flounder you are keeping.

My regular water change routine is at least 25% biweekly to maintain water quality. I do not recommend changing the water frequently, as this could negatively impact the aquarium’s cycle.

Breeding Freshwater Flounder

Breeding freshwater flounders can be challenging, especially since breeding them in captivity is uncommon. There is limited information available on breeding them in aquariums.

In their natural habitats, they tend to breed during spring and summer months. Some species move from brackish water to freshwater to spawn their eggs.

Their spawning process involves females releasing eggs and the males fertilizing them externally. The fertilized eggs are adhesive and will attach to the spawning surface. When the fry hatch, they feed on zooplankton.

How Much Does a Freshwater Flounder Cost?

The cost of a freshwater flounder varies based on several factors, such as size, rarity, and where you buy it. Breeding difficulty also significantly influences the cost of these fish.

Generally, these flatfish are moderately priced, making them accessible to many aquarium enthusiasts. Prices can range from $10 to $30 per fish, depending on the specific type and its availability in the market.

Local pet stores and online retailers are common places to buy freshwater flounders.

The price in local stores can be higher due to the overhead costs of running a physical shop. However, they are still your best go-to since you can visit the physical store to see how the fish are kept and maintained.

Online stores might offer lower prices, but remember to consider shipping costs, which can add to the overall expense. It’s crucial to buy from reputable sources to ensure the health and quality of the fish.

Additional costs also come with owning a freshwater flounder. This includes the aquarium, filtration system, and proper diet, which are essential for their well-being.

These expenses can add up, so it’s important to factor them into your budget before purchasing a flounder. Keeping these expenses in mind will help ensure a healthy environment for your fish.

Recommended Tankmates for a Freshwater Flounder

Freshwater flounders are generally peaceful and can coexist well with other non-aggressive fish.

Ideal tankmates include small, peaceful fish such as discus, neon tetras, guppies, zebrafish, Cherry Barbs, and White Cloud Mountain Minnows. These fish add aesthetic to the aquarium without disturbing the flounders​​.

However, it’s important to be cautious with certain species. Freshwater flounders are bottom dwellers and might prey on very small, slow-moving fish.

Additionally, large or aggressive fish, such as catfish and bass species, and certain brackish water species, like Spotted Scat and Sailfin Molly, should be avoided.

These fish may not be compatible with freshwater flounders due to different water requirements or aggressive behavior​​​​.

Corydoras catfish, danios, tetras, rasboras, and small gouramis are also suitable tankmates, as they are generally peaceful and won’t compete with the flounder for food or territory​​.

Regarding invertebrates, snails and shrimps are not recommended tankmates as they can become quick snacks for flounders.

Creating a community tank involves choosing fish species with similar water parameter needs and temperaments, providing ample hiding places and space, and maintaining a suitable number of fish based on the tank size.

It’s also important to gradually introduce new tankmates, monitor their behavior, and promptly address any signs of aggression.

With their unique physiques and needs, freshwater flounders are truly astonishing species. Do you have more questions about these flatfish marine creatures? Let us know in the comments below!