Can Altum Angel and Freshwater Flounder Live Together?

Altum Angel is a moderately assertive species (5/10), while Freshwater Flounder is peaceful (2/10). This modest difference means Altum Angel may occasionally assert dominance over Freshwater Flounder.

In terms of spatial distribution, Altum Angel prefers the Middle (Open Water) zone, whereas Freshwater Flounder occupies the Bottom (Substrate) zone. Because they use different levels of the water column, territorial disputes are less likely and the tank will appear more balanced.

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

There is no temperature overlap between Altum Angel (27–32°C) and Freshwater Flounder (20–26°C). This fundamental mismatch makes long-term cohabitation impractical.

The pH requirements of Altum Angel (4.5–6.5) and Freshwater Flounder (7.5–8.5) do not overlap, making it impossible to satisfy both species simultaneously.

Hardness requirements are incompatible: Altum Angel needs 1–5 dGH while Freshwater Flounder requires 10–25 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 Altum Angel and Freshwater Flounder together, plan for an aquarium of at least 400 litres with a minimum length of 120 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), Leaf Litter/Blackwater, Plants - Floating. Meeting these requirements will help both species thrive.

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

Altum Angel and Freshwater Flounder 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.