Can Dusky Millions Fish and Northern Mountain Swordtail Live Together?
Keeping Dusky Millions Fish and Northern Mountain Swordtail together is possible but requires caution due to mismatched flow preferences. Provide a spacious tank of at least 110 litres with plenty of hiding spots to reduce stress. Monitor behaviour closely, especially during the first few weeks.
At a Glance
Can your tank handle Dusky Millions Fish and Northern Mountain Swordtail?
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.
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Behaviour & Temperament
Dusky Millions Fish is a peaceful species (2/10), while Northern Mountain Swordtail is generally mild-mannered (3/10). This modest difference means Northern Mountain Swordtail may occasionally assert dominance over Dusky Millions Fish.
Dusky Millions Fish and Northern Mountain Swordtail 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.
Worth noting: Northern Mountain Swordtail is a known jumper, so a tight-fitting lid is essential.
To improve the chances of a peaceful coexistence in this community tank: Keep fin nippers in larger groups to distribute nipping behaviour among conspecifics rather than tank mates.
Water Parameters
The safe temperature window for both species falls between 20°C and 24°C. Aim for the midpoint of this range (around 22.0°C) for optimal comfort.
Both fish can tolerate a pH between 7 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 8–20 dGH. Regular testing will ensure conditions stay within this range.
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Tank Setup
To house Dusky Millions Fish and Northern Mountain Swordtail 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.
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: Plants - Densely covered, Plants - Floating, Established Algae (Otocinclus). Meeting these requirements will help both species thrive.
Note a potential conflict in setup requirements: Dusky Millions Fish is a plant destroyer, which conflicts with Northern Mountain Swordtail's requirement for live plants. Compromise where possible or prioritise the more critical need.
Dusky Millions Fish prefers Low (Still Water) flow while Northern Mountain Swordtail needs High (River/Stream) flow. Consider positioning filter outlets and using baffles to create zones of different current intensity within the same tank.
Suitable Tank Sizes
Standard aquarium sizes large enough for both Dusky Millions Fish and Northern Mountain Swordtail.
30 Gallon Breeder
40 Gallon Breeder
55 Gallon Standard
60 Gallon Breeder
75 Gallon Standard
90 Gallon Standard
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Frequently Asked Questions
Can Dusky Millions Fish and Northern Mountain Swordtail live together?
They can coexist, but the pairing requires careful monitoring. Provide ample space (at least 110 litres), plenty of hiding spots, and watch for signs of stress or aggression.
What size tank do Dusky Millions Fish and Northern Mountain Swordtail 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 Dusky Millions Fish and Northern Mountain Swordtail together?
Keep the aquarium between 20°C and 24°C. A target of around 22.0°C sits comfortably within both species' preferred ranges.
Are Dusky Millions Fish or Northern Mountain Swordtail aggressive?
Dusky Millions Fish is peaceful (2/10) and Northern Mountain Swordtail is generally mild-mannered (3/10). Their similar temperaments generally lead to a peaceful coexistence.
What pH do Dusky Millions Fish and Northern Mountain Swordtail need?
Both species overlap in the 7–8 pH range. Consistency is key — avoid sudden swings, and test regularly to stay within this window.
Will Dusky Millions Fish nip Northern Mountain Swordtail's fins?
Dusky Millions Fish is a known fin nipper. If Northern Mountain Swordtail has long or flowing fins, there is a real risk of fin damage. Keeping Dusky Millions Fish in a larger group can help redirect nipping behaviour toward conspecifics.
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