Molly — Tank Size, Adult Size, Lifespan & Full Care Guide
Poecilia sphenops
Care requirements at a glance
| Scientific name | Poecilia sphenops |
|---|---|
| Group | Livebearer |
| Adult size | 4 in (10.2 cm) |
| Minimum tank | 20 gallons |
| Temperature | 72-82°F |
| pH | 7.5-8.5 |
| Hardness | 15-30 dGH |
| Temperament | Peaceful |
| Social | Group (3+) |
| Diet | Omnivore |
| Swim level | All |
| Care level | Easy |
| Lifespan | ~4 years |
How big do Mollies get? Adult size explained
Mollies are a mid-size species. This is the point where tank footprint starts to matter more than raw gallons — a long tank beats a tall one of the same volume. Adult mollies reach 4.0 inches (10.2 cm). Fish you buy are almost always juveniles at roughly 1.8″, so plan the tank around the adult number, never the size in the bag.
| Stage | Typical length |
|---|---|
| Size sold in stores | about 1.8″ (4.6 cm) |
| Half-grown | 3.0″ (7.6 cm) at roughly 8 months |
| Full adult size | 4.0″ (10.2 cm) by 14 months |
| Sexual maturity | usually before full size, around 6–8 months |
Growth rate depends mostly on feeding quality and water changes in the first six months. Stunted growth from poor water does not make the fish 'stay small' safely — internal organs keep growing, which shortens lifespan considerably.
Molly tank size: minimum vs recommended
The 20-gallon figure is a floor, not a target. It assumes a cycled, filtered, planted tank and an owner who does weekly water changes.
Remember that the minimum tank and the minimum group interact. Mollies need at least 3 individuals, so the real question is not “will one fit in 20 gallons” but “will 3 of them fit”. That is why we recommend 40 gallons as the practical starting point.
| Tank size | Mollies it supports | Verdict |
|---|---|---|
| 5 gallons | — | Too small |
| 10 gallons | — | Too small |
| 20 gallons | 4 in one group | Workable minimum |
| 29 gallons | 6 in one group | Workable minimum |
| 40 gallons | 8 in one group | Comfortable |
| 55 gallons | 9 in one group | Comfortable |
| 75 gallons | 9 in one group | Comfortable |
Numbers assume mollies are the main occupants with light additional stocking, a filter rated for the tank, and weekly 25–30% water changes. Use the stocking calculator to model a mixed community.
Molly group size: how many should you keep together?
Mollies are a shoaling species. In groups below 3 they show measurable stress behaviour: faded colour, hiding, refusing food and darting at the glass. This is not a preference — it is how the species regulates its own sense of safety.
Minimum: 3. Better: 5–6. Colour and natural behaviour improve noticeably once the group passes about 7. If your budget forces a choice between more species and bigger groups, always choose bigger groups — a tank with two well-sized shoals looks far better than one with six lonely singletons.
Molly water parameters
| Parameter | Range | Notes |
|---|---|---|
| Temperature | 72–82°F (22–28°C) | aim for 77°F and keep it stable |
| pH | 7.5–8.5 | target 8.0; stability beats the exact number |
| Hardness (GH) | 15–30 dGH | moderate to hard |
| Ammonia (NH₃) | 0 ppm | any reading means the tank is not cycled |
| Nitrite (NO₂) | 0 ppm | any reading is an emergency — water change now |
| Nitrate (NO₃) | under 20–40 ppm | controlled by water changes and live plants |
This species tolerates — and in fact prefers — harder water, which makes it a good match for most municipal tap water in the UK, US Midwest and much of Europe.
Stability matters more than perfection. A tank sitting steadily at pH 8 is healthier than one swinging between 7.5 and 8.5 every week. Test your tap water first, then choose fish that match it, rather than buying the fish and fighting the water afterwards.
Ideal molly tank setup
Prefer hard, alkaline water (pH 7.5-8.5) and temperatures 75-82°F. A 20-gallon tank with a trio and some peaceful tankmates works well. Add floating plants and open swimming space.
| Substrate | Dark substrate brings out colour; the choice is otherwise up to you. |
|---|---|
| Planting | Plant the back and sides, leave open swimming space down the middle. |
| Flow | Moderate flow suits this species well. |
| Lighting | Moderate. Floating plants diffuse the light and make shy fish bolder. |
| Lid | Essential. This species jumps, especially in the first weeks after introduction. |
What do Mollies eat? Diet and feeding
Omnivores that need both protein and plant matter. Flakes, pellets, blanched zucchini, and spirulina. Feed 1-2 times daily. They benefit from a touch of salt in the water.
The single most common feeding error is quantity, not quality. Mollies have a stomach roughly the size of their eye. Feed what disappears in about two minutes, once a day, and the tank stays clean.
Staple: quality flake or micro-pellet as the staple.
- Frozen daphnia or brine shrimp twice a week
- Blanched vegetable matter once a week
- Occasional live food for conditioning
| Frequency | What to do |
|---|---|
| Daily | One small feeding of quality flake or micro-pellet as the staple. Everything eaten within 2 minutes. |
| 2–3× weekly | Rotate in a second food type for variety and micronutrients. |
| Weekly | One fasting day gives the gut time to clear. Skip this for fry and for shrimp. |
Molly behaviour and temperament
Mollies are peaceful and will not start trouble. Their weakness is the opposite — they are easily bullied or outcompeted for food by faster or bolder species. They spend most of their time in the all levels of the tank.
Signs of a settled, healthy specimen: strong colour, fins held open, active foraging, and a normal reaction to your hand near the glass. Signs of a stressed one: clamped fins, faded colour, hovering in a corner, gasping at the surface, or hiding constantly during daylight hours.
How long do Mollies live?
| Factor | Effect on lifespan |
|---|---|
| Stable, cycled water | Adds years. The most common cause of early death in mollies is ammonia exposure in an uncycled tank. |
| Correct temperature | Running warmer than 82°F speeds metabolism and shortens life; running cold invites disease. |
| Varied diet | Single-food diets cause organ problems and colour loss over time. |
| Right group size | Chronic stress from wrong social setup measurably shortens lifespan. |
| Genetics | Mass-bred stock from large farms often lives noticeably shorter than quality or local stock. |
Molly tank mates: what works and what does not
Compatibility is decided by four things, in this order: water parameters (72–82°F, pH 7.5–8.5), adult size, temperament, and swimming level. A fish that matches on three and fails on one is not a match.
Good tank mates
Check any pairing in the compatibility tool →
Breeding mollies
Livebearers that produce 20-60 fry every 60-70 days. Females can store sperm and produce multiple broods from one mating. Provide dense plants for fry cover.
If you want fry to survive, the practical route is a separate, gently filtered grow-out tank. In a community tank almost every egg or fry becomes food, which is normal and not a sign that anything is wrong.
Male vs female mollies
Males are smaller (3") with a larger dorsal fin and gonopodium. Females are larger (4.5") with a rounded dorsal fin and gravid spot. Males are more active and colorful.
Molly health, diseases and prevention
| Problem | What you see | What to do |
|---|---|---|
| Ich (white spot) | Salt-grain white dots, flicking against decor | Raise temperature slowly to 82°F and treat; aquarium salt is usually tolerated. |
| Fin rot | Ragged, receding or blackened fin edges | Almost always water quality. Increase water changes before reaching for medication. |
| Bloat / constipation | Swollen belly, stringy white waste | Fast 2–3 days, then feed blanched deshelled pea or daphnia. |
| Bacterial infection | Red streaks, ulcers, lethargy | Quarantine and treat with a broad-spectrum antibacterial; review water quality. |
| Stress / chronic hiding | Faded colour, clamped fins | Check parameters, group size and tank mates before medicating. |
Prevention beats treatment every time: quarantine new arrivals for two weeks, keep the tank understocked rather than at maximum capacity, and do a consistent weekly water change instead of a large emergency one every month.
First 30 days with Mollies: a practical checklist
Most losses happen in the first month, and almost all of them are avoidable. This is the schedule we recommend.
| Before you buy | Do not add fish yet. The tank must be fully cycled: ammonia 0, nitrite 0, nitrate present. A cycled tank is the single biggest predictor of whether these animals survive. |
|---|---|
| Day 1 — acclimation | Float the bag 20 minutes to match temperature, then drip acclimate over 45–60 minutes. Never pour store water into your tank. |
| Days 1–3 | Lights off or dimmed. Do not feed on day one. Expect hiding and pale colour; this is normal. |
| Days 4–14 | Test ammonia and nitrite every second day — new stock is the classic trigger of a mini-cycle. Feed lightly. Watch that every individual is eating. |
| Days 14–30 | Colour should be full and behaviour settled. This is the window where ich typically shows up — check body condition from above, not just from the side. |
| After day 30 | Now you can add the next species. Adding stock in stages rather than all at once is what keeps the filter ahead of the bioload. |
Common mistakes when keeping Mollies
- Buying 1 instead of 3 — The most common mistake with this species. An under-sized group is a permanently stressed group, and stress is what makes fish susceptible to everything else.
- Buying juveniles for a small tank — Store specimens are a fraction of the 4.0″ adult size. The tank does not grow with them.
- Adding stock to an uncycled tank — Ammonia burns gills permanently. Cycle first, always — see our tank cycling guide.
- Overfeeding — Uneaten food becomes ammonia. Two minutes, once a day is the rule.
- Chasing pH with chemicals — pH adjusters cause swings that are far more dangerous than a stable pH slightly outside 7.5–8.5.
- Skipping quarantine — Two weeks in a separate tank prevents most disease outbreaks in an established display tank.
What does a molly setup cost?
| Item | Typical cost |
|---|---|
| Aquarium, 40 gal | $108 |
| Filter | $45 |
| Heater | $30 |
| Light | $55 |
| Substrate & decor | $46 |
| Liquid test kit | $32 |
| 3× molly | $24 |
| Estimated total | $340 |
Indicative US retail prices for a new setup. Running cost is modest: mostly electricity for the heater and filter, plus food and a water conditioner. Buying a larger tank up front is almost always cheaper than upgrading twice.
Molly compatibility: verdict per tank mate
Each pair below is a full comparison of temperature, pH, adult size and temperament. Green means compatible, amber means workable with conditions, red means avoid.
Open the full compatibility matrix
Molly FAQ
What size tank do mollies need?
The minimum tank size for mollies is 20 gallons (75 litres). Because they need a group of at least 3, we recommend 40 gallons or larger for a healthy long-term setup.
How big do mollies get?
Adult mollies reach 4.0 inches (10.2 cm). Store specimens are usually juveniles at around 1.8 inches, so always plan the tank around the adult size.
How long do mollies live?
Mollies typically live about 4 years in a well-maintained aquarium. Stable water quality and a varied diet are the two biggest factors in reaching the upper end of that range.
How many mollies should I keep together?
Keep at least 3 together. Groups of 5 to 6 show noticeably better colour and more natural behaviour. Below 3 they become stressed, hide and lose colour.
What water parameters do mollies need?
Keep mollies at 72–82°F (22–28°C), pH 7.5–8.5, and hardness 15–30 dGH. Ammonia and nitrite must both read zero at all times.
Are mollies good for beginners?
Yes. Mollies are rated easy to keep and are a solid choice for a first tank, provided it is fully cycled before they go in.
What do mollies eat?
Omnivores that need both protein and plant matter. Flakes, pellets, blanched zucchini, and spirulina. Feed 1-2 times daily. They benefit from a touch of salt in the water.
Do mollies need a heater?
Yes. Mollies need 72–82°F, which is above typical room temperature. An adjustable heater sized to the tank is essential — temperature swings cause more disease than a slightly wrong average.
Do mollies need a filter?
Yes. A filter houses the bacteria that convert ammonia to nitrate. Choose a filter rated for at least your tank volume, ideally a size up.
Can mollies live alone?
They can survive alone but should not be kept that way. Mollies are a shoaling species and a lone individual is a chronically stressed one, which shortens its life.
What fish can live with mollies?
Good tank mates include Guppy, Platy, Swordtail, Dwarf Gourami, Pearl Gourami, Honey Gourami. Match water parameters first, then temperament, then adult size — anything that can fit them in its mouth eventually will.
Are mollies aggressive?
No. Mollies are peaceful and will not harass other fish. The risk runs the other way — they are easily bullied or outcompeted for food.
How do you tell male from female mollies?
Males are smaller (3") with a larger dorsal fin and gonopodium. Females are larger (4.5") with a rounded dorsal fin and gravid spot. Males are more active and colorful.
Do mollies breed in a home aquarium?
Livebearers that produce 20-60 fry every 60-70 days. Females can store sperm and produce multiple broods from one mating. Provide dense plants for fry cover.
Why are my mollies hiding or losing colour?
In order of likelihood: the tank is not fully cycled (test ammonia and nitrite), the group is too small, the temperature is unstable, or a tank mate is harassing them. Fading colour is the earliest visible stress signal in this species — test the water before treating for anything.
How much does a molly setup cost?
Expect roughly $335 for a complete new 40-gallon setup including tank, filter, light, substrate, a test kit and the animals themselves. Buying the right size once is cheaper than upgrading twice.