Can Penguin Tetra and Regan's Julie Live Together?
Penguin Tetra and Regan's Julie 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 Penguin Tetra and Regan's Julie?
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
Penguin Tetra is a peaceful species (2/10), while Regan's Julie is semi-aggressive (7/10). This notable difference means Regan's Julie may occasionally assert dominance over Penguin Tetra.
Penguin Tetra and Regan's Julie both frequent the 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: Penguin Tetra is a known jumper, so a tight-fitting lid is essential.
Water Parameters
The safe temperature window for both species falls between 23°C and 27°C. Aim for the midpoint of this range (around 25.0°C) for optimal comfort.
The pH requirements of Penguin Tetra (5.5–7.5) and Regan's Julie (8–9) do not overlap, making it impossible to satisfy both species simultaneously.
For general hardness, the shared comfort zone is 10–15 dGH. Regular testing will ensure conditions stay within this range.
Tank Setup
To house Penguin Tetra and Regan's Julie 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: Plants - Densely covered, Plants - Floating, Sand (Sifters). 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.
Why This Pairing Usually Fails in Practice
Penguin Tetra and Regan's Julie 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 Penguin Tetra and Regan's Julie.
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 Penguin Tetra and Regan's Julie live together?
No. This combination is not recommended. Key incompatibilities — such as critical behavioural conflicts — make cohabitation unsafe.
What size tank do Penguin Tetra and Regan's Julie 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 Penguin Tetra and Regan's Julie together?
Keep the aquarium between 23°C and 27°C. A target of around 25.0°C sits comfortably within both species' preferred ranges.
Are Penguin Tetra or Regan's Julie aggressive?
Penguin Tetra is peaceful (2/10) and Regan's Julie is semi-aggressive (7/10). The gap in aggression levels means the calmer fish may experience stress from the more assertive one.
What pH do Penguin Tetra and Regan's Julie need?
Their pH requirements do not overlap. Penguin Tetra prefers 5.5–7.5, while Regan's Julie needs 8–9. This makes them incompatible in the same water chemistry.
Will Penguin Tetra nip Regan's Julie's fins?
Penguin Tetra is a known fin nipper. If Regan's Julie has long or flowing fins, there is a real risk of fin damage. Keeping Penguin Tetra in a larger group can help redirect nipping behaviour toward conspecifics.
How do I manage Regan's Julie's territorial behaviour?
Provide line-of-sight breaks using rocks, driftwood, and dense planting. A larger tank gives Regan's Julie 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 6, 2026
- Last updated
- May 6, 2026
- Issues or corrections?
- Contact the editorial team
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