Molly fish (Poecilia spp.) are popular in freshwater aquariums due to their hardiness, peaceful nature, and vibrant appearance. Among the various molly types, Sailfin and Shortfin (also called Common) Mollies are two of the most recognized. While both make excellent tank mates and are livebearers, there are several key differences between them in terms of appearance, size, behavior, and care needs. This article breaks down the major distinctions to help you choose the right molly for your tank.
Quick Comparison Table
| Feature | Sailfin Molly | Shortfin (Common) Molly |
|---|---|---|
| Body Shape | Longer and deeper body | Shorter and slimmer body |
| Dorsal Fin | Large, flowing “sail”-like fin | Small, typical-sized dorsal fin |
| Size | Up to 6 inches (15 cm) | Up to 4 inches (10 cm) |
| Variants | Sailfin, Lyretail Sailfin, Marble Sailfin | Black, Dalmatian, Gold Dust, etc. |
| Swimming Space | Needs more open space | More adaptable to smaller tanks |
| Breeding | Larger fry batches, longer gestation | Smaller batches, faster recovery |
| Behavior | Peaceful but can be more dominant | Generally peaceful |
| Tank Size | Minimum 30 gallons | Minimum 20 gallons |
Body Size and Shape
Sailfin Mollies have a noticeably larger and more elongated body, with a higher back and more robust frame. They can grow up to 5–6 inches long, especially males with full-grown dorsal fins.
Shortfin Mollies, as the name suggests, are more compact and slender, reaching around 3–4 inches in length.
Dorsal Fin Shape
This is the most striking difference.
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Sailfin Molly: Possesses a large, fan-shaped dorsal fin that can extend halfway or more down the back. Males often flare this fin during mating or when asserting dominance.
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Shortfin Molly: Has a typical dorsal fin that stays flat against the body, making it less flashy but more streamlined.
Note: The extended dorsal fin in Sailfins is often a deciding factor for aquarists looking for showy or ornamental fish.
Color and Variants
Both types come in a wide variety of colors and patterns:
Sailfin Variants:
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Silver Sailfin
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Marbled Sailfin
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Gold Dust Sailfin
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Lyretail Sailfin (with forked tails)
Shortfin Variants:
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Black Molly
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Dalmatian Molly
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Golden Molly
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Balloon Molly (a shortfin variant with a rounded belly)
Color variety is not a differentiator, but the sailfin’s size and shape often make its colors more pronounced.
Behavior and Temperament
Both Sailfin and Shortfin Mollies are peaceful and community-friendly, but Sailfin males may show more territorial or dominant behavior, especially during mating displays. Their large size may intimidate smaller tank mates, even if unintentional.
Shortfins are typically less aggressive and more relaxed, making them ideal for community tanks with small fish.
Tank Size Requirements
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Sailfin Mollies need more swimming room due to their size and active nature. A minimum of 30 gallons is recommended for a small group.
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Shortfin Mollies can thrive in 20-gallon tanks, making them more suitable for beginners or smaller setups.
In either case, horizontal swimming space is important.
Breeding Behavior
Both types are livebearers, but there are subtle differences:
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Sailfin Females are larger and can produce more fry per batch (sometimes 60–100+), with slightly longer gestation periods.
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Shortfin Females may have smaller broods (30–50) and tend to recover and breed again more quickly.
Both can store sperm for multiple births after a single mating, so be prepared for fry if males and females are kept together.
Feeding and Diet
Both types have identical dietary needs:
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Omnivorous: Flake food, algae wafers, blanched vegetables, and live or frozen protein (brine shrimp, daphnia)
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Sailfins may need slightly more food due to their larger size.
Care Difficulty
Neither fish is particularly difficult to care for, but:
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Sailfins need more space, better filtration, and can be slightly more sensitive to water quality due to their size.
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Shortfins are more forgiving, making them slightly better for beginners or smaller setups.
Both benefit from:
Hard, alkaline water
Stable temperatures (75–80°F / 24–27°C)
Weekly water changes
A cycled tank with good filtration
Which One Is Right for You?
| Your Aquarium Goal | Best Choice |
|---|---|
| Small tank (20–30 gallons) | Shortfin Molly |
| Showy centerpiece fish | Sailfin Molly |
| Beginner-friendly care | Shortfin Molly |
| Breeding project | Either (Sailfins produce more) |
| Peaceful community tank | Shortfin Molly |
| More dramatic fin display | Sailfin Molly |
Final Thoughts
Whether you choose the elegant Sailfin Molly or the more compact Shortfin Molly, both are wonderful additions to a freshwater aquarium. Understanding their differences in size, space needs, and fin shape can help you make the best decision for your setup. With proper care, both types will reward you with lively personalities, graceful swimming, and frequent breeding activity.
