Freshwater Flounder Care: Brackish Tank, Size & Diet

Freshwater flounder are small, peaceful flatfish that spend the day buried in sand and hunt at night. Despite the trade name, the fish sold as “freshwater flounder” are really brackish water fish: they cope with freshwater as juveniles but do best in low-salinity brackish water as adults.

Can flounder live in freshwater? Not long term. Young fish can live in freshwater for a while, but adults need at least low-end brackish water (specific gravity about 1.005 to 1.010) to stay healthy. Plan for a fine-sand tank of 30 gallons or more, a temperature of roughly 68 to 79°F, a pH of about 7.5 to 8.5, and live or frozen meaty foods. Most live 2 to 4 years in aquariums, and wild fish can reach 7.

This guide covers which species you’re actually buying, tank setup, salinity, feeding, tank mates and breeding.

Freshwater Flounder Facts & Overview

“Freshwater flounder” is a trade name, not a single species. Fish sold under it are normally one of two small American soles, and both are estuary fish:

• Trinectes maculatus (hogchoker) is the one most often sold. It lives in estuaries and bays from Massachusetts to Florida and around the Gulf of Mexico. Juveniles move up rivers into freshwater, then return to brackish water as they mature. Adults reach about 6 inches (15 cm), and some sources say up to 8 inches.
• Achirus lineatus (lined sole) ranges from Florida and the northern Gulf of Mexico south to northern Argentina. It prefers brackish or even saltier lagoons and estuaries, generally above 15 ppt. Common size is about 6 to 7 inches, and some reports go larger.
• Catathyridium jenynsii is a true freshwater tonguesole from the Paraná and Uruguay river systems in South America. It is rarely, if ever, sold in the aquarium trade, so a “freshwater flounder” at your local shop is almost certainly not this one.

Both common species are wild-caught. They are not bred commercially, so availability varies and wild fish can arrive thin or stressed.

How do freshwater flounder behave?

Freshwater flounder are shy, nocturnal ambush predators. They lie flat or half-buried for most of the day, often with only the eyes showing, and become active after lights-out. Expect to see little of them in a bright community tank.

They are peaceful, but they will eat anything small enough to fit in their mouths, including fry and small shrimp.

Related Reading: The Complete Freshwater Puffer Fish Care Guide

What do freshwater flounder look like?

Both species are right-sided flatfish. They hatch looking like ordinary fish, with an eye on each side of the head. As they mature, the left eye migrates to the right side, so both eyes end up on the upper surface and the pale underside rests on the substrate.

Their camouflage is excellent. The hogchoker is a grayish-brown fish with thin, dark vertical stripes and a rounded body. The lined sole is lighter brown with dark lines and blotches. The mouth and gills sit on the underside.

What water conditions do freshwater flounder need?

Adults need brackish water. A specific gravity of 1.005 to 1.010 (roughly 7 to 14 ppt) is the range hobbyists most often report for hogchokers, and they tolerate somewhat higher. Lined soles come from saltier water and do better at the upper end or above. Juveniles can sit in freshwater for a short time, but fish kept in freshwater long term tend to decline within a couple of years.

Do not rely on the old advice that they must eventually move to full marine conditions. Hogchokers do not need a reef-strength tank.

What is the right tank setup?

The substrate matters more than anything else. Use 2 inches or more of fine, soft sand, such as play sand or aragonite sand. Gravel and sharp-edged sand scrape the underside, leading to sores and infections. Add driftwood and smooth rock for cover, keeping the middle of the floor open for the fish to bury.

Plants that tolerate brackish water, such as Anubias Nana and Java Fern, work well attached to wood.

• Temperature: about 68 to 79°F (20 to 26°C). Sources vary, so aim for the low-to-mid 70s.
• pH: 7.5 to 8.5.
• Hardness: hard water, about 10 to 25 dGH.
• Flow: gentle. These are bottom-hugging ambush fish, not current-lovers.

What size aquarium do freshwater flounder need?

Plan on at least 30 gallons with a long footprint (36 inches or more), because floor space matters more than height. Some keepers recommend 55 gallons for a full-grown adult, especially if you want tank mates. A 10-gallon tank is too small once the fish passes its juvenile stage.

What are good tank mates?

Choose calm, brackish-tolerant fish that stay out of the sand and do not rush the flounder at feeding time. Spiny eels and violet gobies are often suggested. Avoid ornamental shrimp and very small fish, which will be eaten, and avoid fast, boisterous fish that outcompete the flounder for food or keep it hiding.

Can you keep more than one freshwater flounder?

Yes, in a tank large enough to give each fish its own patch of sand. Most keepers stay with one or two, since the fish is not an active display species.

What do freshwater flounder eat?

In the wild they pick worms, small crustaceans and insect larvae out of the sand. In an aquarium, feed sinking meaty foods: bloodworms, blackworms, krill, chopped shrimp and small pieces of fish. Feed shortly after the lights go out.

Many wild-caught fish refuse flakes and pellets at first. If yours won’t eat, try live blackworms or ghost shrimp, or dangle a piece of shrimp on feeding tongs. Fish that stay thin after a few weeks usually need live food to get started.

How hard are freshwater flounder to care for?

They are intermediate fish. The care itself is simple, but the salinity, soft sand and feeding all have to be right from the start. Common mistakes are:

• Keeping them in plain freshwater because of the name.
• Using gravel or coarse substrate.
• Housing them with fast fish that take all the food.
• Relying on flakes for a wild-caught fish that has not learned to eat them.

Watch for red or raw patches on the underside, which point to a substrate or water problem, and for a flounder that stays thin and never feeds.

Can you breed freshwater flounder in an aquarium?

Not in practice. Hogchokers go to saltier water to spawn, and there are no reliable reports of aquarium breeding. In the wild, spawning takes place in estuaries during the warmer months, and the larvae drift before settling and changing shape.

Are freshwater flounder right for your tank?

They suit keepers who already run, or want to run, a low-salinity brackish tank and who like unusual, low-key fish. They do not suit a freshwater community tank, a bright tank with busy fish, or anyone who wants a colorful centerpiece.

Frequently Asked Questions

Can flounder be in freshwater?

Not long term. Juvenile hogchokers tolerate freshwater for a while, but adults need at least low-end brackish water (specific gravity about 1.005 or higher) to stay healthy and reach a normal lifespan. Fish kept in plain freshwater tend to decline within a couple of years.

How big do freshwater flounder get?

About 6 inches for the hogchoker, with some sources reporting up to 8. The lined sole commonly reaches around 6 to 7 inches, and sometimes more.

How long do freshwater flounder live?

Aquarium fish typically live 2 to 4 years. Wild hogchokers have been reported to live up to 7 years.

What do freshwater flounder eat?

Live and frozen meaty foods that sink, such as blackworms, bloodworms, krill and shrimp. Feed shortly after lights-out, since they hunt in the dark.

Are freshwater flounder good for beginners?

Not really. The basic care is easy, but they need brackish water, soft sand and a feeding plan for wild-caught fish. They are better for keepers with some experience.

What tank size do freshwater flounder need?

At least 30 gallons with a long footprint (36 inches or more) for an adult, and 55 gallons if you want tank mates. A 10-gallon tank only works for a small juvenile.