Can Betta Macrostoma and Cairns Rainbowfish Live Together?
Betta Macrostoma and Cairns Rainbowfish are not recommended as tank mates due to conflicting pH requirements. Housing these species together is likely to result in stress, health problems, or direct harm to one or both fish.
At a Glance
Can your tank handle Betta Macrostoma and Cairns Rainbowfish?
Compatibility is only part of the answer. Test both fish with your real tank size, current stock, and maintenance needs to see if you have enough space and a safe stocking level before you add them.
Behaviour & Temperament
Betta Macrostoma is a moderately assertive species (6/10), while Cairns Rainbowfish is peaceful (2/10). This notable difference means Betta Macrostoma may occasionally assert dominance over Cairns Rainbowfish.
Betta Macrostoma and Cairns Rainbowfish 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.
Additional behavioural considerations to keep in mind: Hyperactive swimmers can stress shy, slow-moving species through constant motion.
Worth noting: Betta Macrostoma is a known jumper, so a tight-fitting lid is essential.
Water Parameters
The safe temperature window for both species falls between 20°C and 25°C. Aim for the midpoint of this range (around 22.5°C) for optimal comfort.
The pH requirements of Betta Macrostoma (4–6) and Cairns Rainbowfish (6.5–7.6) do not overlap, making it impossible to satisfy both species simultaneously.
For general hardness, the shared comfort zone is 2–5 dGH. Regular testing will ensure conditions stay within this range.
Tank Setup
To house Betta Macrostoma and Cairns Rainbowfish together, plan for an aquarium of at least 110 litres with a minimum length of 90 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: Leaf Litter/Blackwater, Driftwood (Digestion/Hiding), Plants - Floating, Smooth Gravel (Sensitive Barbels). Meeting these requirements will help both species thrive.
Betta Macrostoma prefers Low (Still Water) flow while Cairns Rainbowfish needs High (River/Stream) flow. Consider positioning filter outlets and using baffles to create zones of different current intensity within the same tank.
Why This Pairing Usually Fails in Practice
Betta Macrostoma and Cairns Rainbowfish 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.
Suitable Tank Sizes
Standard aquarium sizes large enough for both Betta Macrostoma and Cairns Rainbowfish.
30 Gallon Breeder
40 Gallon Breeder
55 Gallon Standard
60 Gallon Breeder
75 Gallon Standard
90 Gallon Standard
Show 10 more tank sizes
Frequently Asked Questions
Can Betta Macrostoma and Cairns Rainbowfish live together?
No. This combination is not recommended. Key incompatibilities — such as critical behavioural conflicts — make cohabitation unsafe.
What size tank do Betta Macrostoma and Cairns Rainbowfish need?
A minimum of 110 litres (tank length at least 90 cm) is recommended. This provides enough space for both species to establish their own areas and reduces the likelihood of territorial disputes.
What water temperature is best for Betta Macrostoma and Cairns Rainbowfish together?
Keep the aquarium between 20°C and 25°C. A target of around 22.5°C sits comfortably within both species' preferred ranges.
Are Betta Macrostoma or Cairns Rainbowfish aggressive?
Betta Macrostoma is moderately assertive (6/10) and Cairns Rainbowfish is peaceful (2/10). The gap in aggression levels means the calmer fish may experience stress from the more assertive one.
What pH do Betta Macrostoma and Cairns Rainbowfish need?
Their pH requirements do not overlap. Betta Macrostoma prefers 4–6, while Cairns Rainbowfish needs 6.5–7.6. This makes them incompatible in the same water chemistry.
How do I manage Betta Macrostoma's territorial behaviour?
Provide line-of-sight breaks using rocks, driftwood, and dense planting. A larger tank gives Betta Macrostoma space to claim a territory without encroaching on the other fish's area. Adding décor that divides the tank into distinct zones works especially well.
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Guidarium Editorial Desk
Reviewed against Guidarium care, stocking, and compatibility standards. Read the editorial policy.
- Last reviewed
- May 3, 2026
- Last updated
- May 3, 2026
- Issues or corrections?
- Contact the editorial team
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