Guppy vs molly: which should you choose?
AquaLog guide · Published
In short: Choose guppies for a smaller tank: they need 40 L (11 US gal) and grow to about 5 cm (2 in). Choose mollies if you have at least 90 L (24 US gal) and hard, alkaline water; they reach about 10 cm (3.9 in) and live longer. Both are peaceful beginner livebearers, and they can share a tank kept at 24–28 °C (75–82 °F) and pH 7.5–8.5.
How do they compare?
| Guppy | Molly | |
|---|---|---|
| Minimum tank | 40 L (11 US gal) | 90 L (24 US gal) |
| Temperature | 22–28 °C (72–82 °F) | 24–28 °C (75–82 °F) |
| pH | 7–8.5 | 7.5–8.5 |
| Hardness | 8–25 dGH | 12–30 dGH |
| Adult size | 5 cm (2 in) | 10 cm (3.9 in) |
| Temperament | Peaceful | Peaceful |
| Group size | 3 or more | 3 or more |
| Care level | Beginner | Beginner |
| Lifespan | 1–3 years | 3–5 years |
| Diet | Omnivore: flake, micro pellets, occasional vegetable matter | Herbivore-leaning omnivore: algae wafers, vegetable flake, greens |
| Where it swims | Top | All levels |
All figures come from our care guide: see guppy and molly.
What is the real difference?
Size and water. An adult molly is twice the length of a guppy and needs more than twice the water: 90 L (24 US gal) against 40 L (11 US gal). Mollies also want harder water. Their range starts at pH 7.5 and 12 dGH, where the guppy's starts at pH 7 and 8 dGH. Guppies are short-lived at 1–3 years; mollies are listed at 3–5.
Choose guppies if…
- Your tank holds less than 90 litres (24 US gal). Guppies need 40 L (11 US gal).
- Your water is only moderately hard. Guppies are listed from pH 7 and 8 dGH.
- You want colour in a small space. Males are small, with a large, colourful tail.
- You want small, peaceful company. Our guppy data lists corydoras, the platy, otocinclus and nerite snails as good tank mates.
Choose mollies if…
- You have at least 90 L (24 US gal) and want a bigger, bolder fish.
- Your tap water is hard and alkaline. Mollies are listed up to 30 dGH, the hardest water of the two.
- You can feed plenty of vegetable food. Mollies lean herbivore: algae wafers, vegetable flake and greens.
- You want a longer-lived fish: 3–5 years against the guppy's 1–3.
Can guppies and mollies live together?
Yes. Their ranges overlap at 24–28 °C (75–82 °F), pH 7.5–8.5 and 12–25 dGH. Both are peaceful, neither lists the other as a fish to avoid, and both list the platy as a good tank mate. Size the tank for the molly: at least 90 L (24 US gal). The aquarium stocking checker runs the same checks for any other species you add.
Check both avoid lists before adding anything else. Both name the betta. The guppy's list adds tiger barbs and fin-nipping tetras, and the molly's adds soft-water species, so a mixed tank stays hard and alkaline.
What should a buyer know about fry?
- Both breed constantly. Our guppy data says a female drops 20–50 fry every month without any help, and mollies have large broods. A mixed-sex tank fills up quickly unless you have a plan for the young.
- Shop females may already be pregnant. Female guppies store sperm for up to six months and can give birth to several broods with no male present. In one study in Trinidad, a single wild-caught female was put alone in each outdoor tank, and 86% of the populations they founded were still going two years later.
- A males-only group avoids fry entirely. A females-only group of guppies can still give birth for months.
- Pick healthy stock. Our guppy data lists inbred, weak stock among its common problems, alongside fin rot and columnaris. Mollies list molly shimmies, columnaris and ich.
Check your pH and hardness before buying: aquarium water parameters explained covers both. If your tap water is hard, fish for hard, alkaline water lists more species that suit it. In AquaLog, the "Will it fit your tank?" check compares either fish with your logged tank's size and latest temperature and pH.