Can Freshwater Flounder and Uaru Live Together?

Freshwater Flounder is a peaceful species (2/10), while Uaru is generally mild-mannered (4/10). This modest difference means Uaru may occasionally assert dominance over Freshwater Flounder.

Freshwater Flounder and Uaru both frequent the Bottom (Substrate) region of the setup. This overlap can increase competition for space, so providing ample room and cover is important to keep both species comfortable.

Worth noting: Freshwater Flounder is nocturnal and will be most active after lights-out, naturally reducing daytime encounters.

There is no temperature overlap between Freshwater Flounder (20–26°C) and Uaru (27–30°C). This fundamental mismatch makes long-term cohabitation impractical.

The pH requirements of Freshwater Flounder (7.5–8.5) and Uaru (5–7) do not overlap, making it impossible to satisfy both species simultaneously.

Hardness requirements are incompatible: Freshwater Flounder needs 10–25 dGH while Uaru requires 1–8 dGH.

Crucially, these species require different water types — one needs freshwater while the other requires brackish conditions. This is a hard barrier to keeping them together.

To house Freshwater Flounder and Uaru together, plan for an aquarium of at least 400 litres with a minimum length of 150 cm. This recommendation combines the stricter species minimums with a community-load allowance so the pairing has realistic long-term space and filtration headroom.

Providing a mix of open swimming space and sheltered areas with rocks, driftwood, or plants will keep both species comfortable and allow natural behaviour.

Specific environmental needs for this combination include: Sand (Sifters), Driftwood (Digestion/Hiding). Meeting these requirements will help both species thrive.

Both species do well with moderate (standard) water movement, so a standard filtration setup rated for the tank volume should suffice.

Freshwater Flounder and Uaru are the kind of pair that can look acceptable in a shop but become a maintenance problem at home. The main risk is not a single number in the chart; it is the way incompatible behavior, water needs, or pressure on space compounds over time once both fish are established in the same tank.