Molly care guide
Also called: Sailfin molly, Black molly, Balloon molly, Dalmatian molly, Lyretail molly, Short-finned molly, Common molly
Mollies are hardy, peaceful livebearers sold as black, dalmatian, balloon and sailfin varieties. They graze algae and suit busy community tanks, but they need room, warm water and hard, alkaline water to stay healthy.
Mollies need a tank of at least 29 gallons (110 L), a group of 3 or more, and water at 24–27°C (75–81°F) with a pH of 7.5–8.5. They grow to about 10 cm (3.9 in) and are peaceful.
Plan a tank with molliesSee all tank mates
Molly quick facts
| Scientific name | Poecilia spp. (mainly P. sphenops, P. latipinna and their hybrids) |
|---|---|
| Family | Poeciliidae |
| Origin | Southern USA and Mexico to northern South America (aquarium mollies are captive-bred, mostly hybrids) |
| Adult size | 10 cm (3.9 in) |
| Minimum tank | 29 gallons (110 L), at least 75 cm (30 in) long |
| Temperature | 24–27°C (75–81°F) |
| pH | 7.5–8.5 |
| Hardness (GH) | 15–30 dGH |
| Group size | 3 or more (1 male, the rest females) |
| Temperament | Peaceful; males fight each other |
| Swims at | Middle |
| Diet | Omnivore |
| Care level | Moderate |
| Lifespan | 3–5 years |
| Safe with shrimp? | No, eats shrimp |
| Safe with plants? | Mostly (nibbles soft plants) |
Tank setup
Plan on at least a 29 gallon (110 L) tank about 30 in (75 cm) long, especially for sailfin mollies, which can reach 4–5 in (10–13 cm). Short-finned mollies can start in 20 gallons (76 L) but outgrow it as the group breeds. Give open swimming room plus plants for fry and females. Mollies make a lot of waste, so use strong filtration.
How many mollies in a tank?
The smallest common tank that suits mollies is a 29 gallon. Here is roughly how many fit in each size when they are the only fish, based on adult size and waste output. Leave room if you want tank mates.
| Tank | Mollies (alone) |
|---|---|
| 3 gallon | Too small |
| 5 gallon | Too small |
| 10 gallon | Too small |
| 15 gallon | Too small |
| 20 gallon | Too small |
| 20 gallon long | Too small |
| 29 gallon | 3–8 |
| 40 gallon | 3–11 |
| 55 gallon | 3–15 |
| 75 gallon | 3–21 |
| 125 gallon | 3–35 |
Numbers are bioload limits only. Males fight, so keep a group of 3 or more with more females than males.
What you need for mollies
Sized for a 29 gallon tank, the smallest common size that suits a group of 3 mollies. For a different tank or a mix of fish, use the stocking calculator for a list sized to your plan.
Tank and equipment
- 29 gallon aquarium (tank or kit) (opens Amazon in a new tab)Bigger tanks are more stable and easier for beginners. How to choose
- Aquarium stand for 29 gallons (opens Amazon in a new tab)The water alone weighs about 242 lb (110 kg), before glass, gravel and rock. How to choose
- Adjustable heater, 100 W (opens Amazon in a new tab)Holds about 78 °F (25.5 °C) in a room at 65 °F (18 °C). How to choose
- Aquarium thermometer (opens Amazon in a new tab)Check it daily: heaters can fail stuck on or off. How to choose
- Hang-on-back filter rated 115–175 GPH (opens Amazon in a new tab)Community tank: 4–6 times the tank volume per hour. How to choose
- About 45 lb of aquarium gravel (opens Amazon in a new tab)Enough for a layer about 2 in (5 cm) deep. Smooth gravel suits most community fish. How to choose
- Water conditioner (dechlorinator) (opens Amazon in a new tab)Removes chlorine and chloramine from tap water. Seachem Prime is a popular, concentrated choice. How to choose
- Liquid test kit (ammonia, nitrite, nitrate, pH) (opens Amazon in a new tab)Liquid kits are more accurate than strips. The API Freshwater Master Test Kit is the common choice. How to choose
- Gravel vacuum (siphon) (opens Amazon in a new tab)Cleans the gravel and does the water change in one go. How to choose
- LED aquarium light, about 30 in long (opens Amazon in a new tab)Match the light to the tank length. Basic LEDs grow easy plants. How to choose
Food and extras for mollies
- Tropical fish pellets (opens Amazon in a new tab)A good staple for medium community fish.
Product links go to Amazon search results. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Disclosure.
Water parameters
Keep mollies warm, at 24–27 °C (75–81 °F), with pH 7.5–8.5 and hard water of about 15–30 dGH. Soft or acidic water is a common reason mollies get sick or start to shimmy. They don't need salt if the water is hard enough, though many are farm-raised in brackish water and tolerate a little. Change about 25% of the water each week.
Best tank mates for mollies
These popular species share mollies' water needs and temperament. See the full list of 72 good matches.
Betta fish
6.5 cm (2.6 in) · 24–28°C (75–82°F)
Nerite snail
2.5 cm (1 in) · 22–27°C (72–81°F)
Guppy
5 cm (2 in) · 22–27°C (72–81°F)
Dwarf gourami
7.5 cm (3 in) · 24–28°C (75–82°F)
Bristlenose pleco
12 cm (4.7 in) · 22–26°C (72–79°F)
Bronze corydoras
7 cm (2.8 in) · 21–26°C (70–79°F)
Angelfish
15 cm (5.9 in) · 24–29°C (75–84°F)
Assassin snail
2.5 cm (1 in) · 22–27°C (72–81°F)
Avoid keeping mollies with
- Cherry shrimp: Molly eats small shrimp like cherry shrimp.
- Common goldfish: They need different temperatures: Molly 24–27°C (75–81°F), common goldfish 15–22°C (59–72°F).
- Fancy goldfish: They need different temperatures: Molly 24–27°C (75–81°F), fancy goldfish 18–23°C (64–73°F).
- Oscar: Oscar can eat mollies: they're small enough to fit in its mouth once it's grown.
- Chinese algae eater: Chinese algae eater is aggressive and will bully or injure peaceful mollies.
- Convict cichlid: Convict cichlid is aggressive and will bully or injure peaceful mollies.
Feeding
Mollies are omnivores that need plenty of plant matter. Feed a vegetable-based flake or pellet as the staple, plus algae wafers, blanched spinach or zucchini, and frozen foods like brine shrimp. They also graze hair algae and biofilm. Long strings of waste hanging from the fish are a sign of overfeeding.
Behavior
Mostly peaceful, but active and curious. Males chase females constantly and sailfin males may spar, so keep two or three females per male. Mollies pick at everything, which can bother slow, long-finned fish, and large mollies eat small shrimp. Good tank mates are other livebearers, corydoras, danios and hard-water tetras or barbs.
Breeding
Mollies breed readily. Gestation takes about 4–8 weeks, and an experienced female can drop 50 or more fry. Adults eat their young, so dense plants or a separate tank improve survival. Short-finned and sailfin mollies interbreed freely, which is how most store varieties were made.
Tips from experienced keepers
- Test your GH before buying. If your tap water is soft, add minerals or pick a soft-water fish instead.
- Balloon mollies have a shortened, rounded body that can cause swimming and digestion problems. Short-finned or sailfin types are hardier.
- Keep two or three females for every male to spread out the chasing.
- Mollies are good hair algae grazers, but they may also nibble soft plants.
- Don't keep mollies with a dwarf shrimp colony. Larger mollies will eat the shrimp.
Frequently asked questions
Do mollies need salt in their tank?
No. Mollies do well in fresh water as long as it is hard and alkaline. They tolerate a little aquarium salt, but hard, mineral-rich water matters far more than salt.
How big do mollies get?
Short-finned and balloon mollies usually reach 2.5–3 in (6–8 cm). Sailfin mollies are bigger, often 4–5 in (10–13 cm), and females grow larger than males.
How many mollies should be kept together?
Start with at least three, ideally one male with two or three females. A larger group spreads out the males' attention and looks more natural.
Sources
We checked these references when writing this page. Values are for healthy long-term keeping, not survival limits. Last reviewed 2026-10-13. See our editorial policy.