Can Goldfish (Comet / Common) and Molly (Sailfin/Black/Dalmatian) Live Together?

Goldfish (Comet / Common) is a peaceful species (1/10), while Molly (Sailfin/Black/Dalmatian) is generally mild-mannered (3/10). This modest difference means Molly (Sailfin/Black/Dalmatian) may occasionally assert dominance over Goldfish (Comet / Common).

Goldfish (Comet / Common) and Molly (Sailfin/Black/Dalmatian) both frequent the Top (Surface) and Middle (Open Water) region of the setup. This overlap can increase competition for space, so providing ample room and cover is important to keep both species comfortable.

The safe temperature window for both species falls between 24°C and 24°C. Aim for the midpoint of this range (around 24.0°C) for optimal comfort.

Both fish can tolerate a pH between 7.5 and 8. Maintaining a stable value within this band is more important than hitting an exact number.

For general hardness, the shared comfort zone is 12–20 dGH. Regular testing will ensure conditions stay within this range.

To house Goldfish (Comet / Common) and Molly (Sailfin/Black/Dalmatian) together, plan for an aquarium of at least 200 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.

Because this pairing requires extra vigilance, include plenty of hiding spots — caves, driftwood, and dense plant clusters — so the less dominant fish can retreat when needed.

Specific environmental needs for this combination include: Sand (Sifters), Smooth Gravel (Sensitive Barbels), Plants - Densely covered, Established Algae (Otocinclus). Meeting these requirements will help both species thrive.

Note a potential conflict in setup requirements: Goldfish (Comet / Common) is a plant destroyer, which conflicts with Molly (Sailfin/Black/Dalmatian)'s requirement for live plants. Compromise where possible or prioritise the more critical need.

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

This pairing usually fails when the tank is treated like a standard mixed community and the caution flags are ignored. Goldfish (Comet / Common) and Molly (Sailfin/Black/Dalmatian) need extra room, more cover, and closer observation during the first few weeks so small aggression, feeding, or territory issues do not snowball into a long-term problem.