Molly

Mollies thrive in hard, alkaline water—not soft community tanks. These active livebearers grow 3–4.5 inches, graze constantly, and breed freely. Fix hardness first; salt is optional, not mandatory.

Who This Fish Is For

Mollies suit beginners and community keepers who already run hard, alkaline water—or who will build hardness with crushed coral or aragonite. They are larger and hungrier than guppies or platies, so they need more swimming room and more waste control.

Skip mollies if your tap is soft and acidic and you refuse to buffer it. Soft water is the main reason mollies shimmy and fail. Skip them in 10-gallon nanos; plan for at least 20 gallons, preferably 30. Balloon mollies carry a compressed-spine deformity and shorter, harder lives—standard short-fin or sailfin lines are the better choice.

If you want colorful livebearers that graze algae, hold their own in a busy community tank, and produce fry regularly, mollies deliver—once the water chemistry fits.

Care Snapshot

Spec	Value
Scientific name	Poecilia sphenops (hybrids common; sailfins often P. latipinna / P. velifera genetics)
Adult size	3–3.5 in standard; 4–4.5 in sailfin lines
Lifespan	3–5 years
Difficulty	Beginner—in hard water
Minimum tank	20 gal (3–4 fish); 30 gal recommended
Temperature	75–82°F (sweet spot 78–80°F)
pH	7.5–8.5
Hardness	Hard (aim ~15–30 dGH)
Temperament	Peaceful; males chase when short on females
Diet	Omnivore, plant-heavy (algae, veg flakes, occasional protein)
Breeding	Livebearer; 20–100+ fry every 4–6 weeks

Tank Setup

Volume: 20 gallons minimum for a small group; 29–40 gallons is more stable for mixed communities. Stock roughly one molly per 5–7 gallons once adults fill out. Sailfins and large hybrids need the higher end.

Filtration and flow: Hang-on-back or canister with moderate flow. Mollies use the current and need good oxygenation. A sponge filter is useful backup bioload capacity if you keep a breeding group.

Heater: Reliable heater sized to the tank. Hold tropical temperatures; avoid swings larger than a couple of degrees during water changes.

Aquascape: Dark gravel or sand, smooth rock and driftwood for grazing surfaces, and tough plants—Java fern, Anubias, crypts, Vallisneria, Amazon swords. Soft stem plants get nibbled. Floating cover (frogbit, water sprite) helps fry hide without blocking all light.

Lid: Not notorious jumpers, but a lid is still good practice in active tanks. Follow a full aquarium setup and filtration plan before stocking.

Salt note: Freshwater is fine when hardness and pH are right. Light salt or mild brackish (about 1.001–1.005 SG) can help some lines or stressed fish. If you salt, use aquarium or marine salt—not table salt—and accept that many plants will suffer. Salt does not fix soft water by itself.

Water Parameters

Mollies fail most often on chemistry, not “difficulty.”

• Temperature: 75–82°F; match new water during changes.
• pH: 7.5–8.5. Below ~7.0 long-term is a stressor.
• Hardness: Hard, mineral-rich water. Soft water is the classic trigger for shimmies (constant trembling, clamped fins, poor swimming).
• Ammonia / nitrite: 0 ppm always.
• Nitrate: Keep under ~40 ppm with weekly changes; adults produce more waste than small livebearers.

Soft tap fixes: Crushed coral or aragonite in the filter or substrate, mineral supplements (e.g., Wonder Shells), or blending with harder water. Test hardness after you buffer so you know what actually moved. See water testing and water sources.

Maintenance: 25–30% weekly water changes, vacuum messy feeding zones, rinse filter media in tank water only. Check pH and hardness when fish look off—do not assume salt first. After buffering with coral or aragonite, retest in a few days—pH and GH can drift as media dissolves. Keep a simple log of temperature, pH, and hardness so you notice slow soft-water creep before shimmies start.

Shimmies protocol: Trembling in place, clamped fins, and weak swimming almost always point at environment. Test hardness and pH, warm toward 80–82°F if the tank is cool, perform a partial change with mineral-rich water, and only then consider a measured salt dose if it fits your setup. Medication is the wrong first move when chemistry is soft and acidic.

Diet and Feeding

Mollies are omnivores with a strong plant bias. Aim for roughly 60–70% vegetable matter.

Staple: Spirulina- or veggie-forward flakes and pellets. Algae wafers fill the gap between meals.

Produce: Blanched zucchini, cucumber, spinach, or similar a few times a week. Clip food so it stays available for grazing; pull leftovers within a day.

Protein: Frozen daphnia, brine shrimp, or bloodworms 2–3 times weekly—not as the main diet. Excess protein fouls water and can cause digestive trouble.

Schedule: 2–3 small feedings daily, only what is finished in 2–3 minutes. Between meals they should pick biofilm and algae off glass and hardscape. Details on variety and portions live in the feeding guide.

Temperament and Tank Mates

Peaceful community fish that use all levels. Males form pecking orders and harass females if ratios are wrong. Keep at least two females per male (three is better).

Good mates: Other livebearers (guppy, platy, swordtail), mid-size tetras and rasboras, Corydoras, bristlenose plecos, Amano shrimp. Compare livebearer options in platy vs molly.

Avoid: Large cichlids and predators, serious fin-nippers, cool-water goldfish, and bettas in the same small, hard-water setup—water and temperament usually favor one species over the other. Neon tetras and tiny rasboras can be outcompeted or intimidated by bold adult mollies.

Use the compatibility checker and tank mate guide when planning stocking density.

Breeding reality: Sex males by the rod-like gonopodium; females keep a fan-shaped anal fin and show a dark gravid spot when close to birth. Gestation is about 4–6 weeks. Fry are larger than guppy fry and survive better if plants are dense. Adults eat fry—that is normal population control. Options: heavy cover (Java moss, guppy grass), a temporary breeding box (stressful—use sparingly), or a separate grow-out tank. Plan for surplus (donation, trade) before a colony explodes. See breeding aquarium fish.

Varieties in the trade: Black, gold, dalmatian, marble, lyretail, and sailfin lines are common. Most pet-store “mollies” are hybrids among P. sphenops, P. latipinna, and P. velifera, which is why adult size and salt preference vary. Give oversized “sailfins” more water volume and do not force them into soft community chemistry.

Common Mistakes

1. Soft, acidic water. Number-one cause of shimmies and slow decline. Buffer hardness before treating with random meds.
2. Salt as a magic fix. Salt can help some setups; it does not replace minerals and stable alkalinity.
3. 10-gallon tanks. Adults are too large and dirty for nanos.
4. One male, one female. Constant harassment stresses the female.
5. Protein-only feeding. Digestion and water quality both suffer.
6. Buying balloon mollies on looks alone. Expect organ crowding, swim issues, and shorter lives.
7. Ignoring fry planning. Healthy groups will breed; have a plan.

Frequently Asked Questions

Do mollies need salt?

Not always. Hard, alkaline water matters more. Salt is optional for some lines, mild brackish goals, or short-term stress support after you fix chemistry.

Why is my molly shimmying?

Usually soft water, low pH, chill, or dirty water. Test hardness and pH first; correct minerals and temperature. Recovery often takes days once conditions improve.

How big do mollies get?

About 3–3.5 inches for common short-fin lines; sailfins around 4–4.5 inches. True giant sailfin genetics can run larger.

Can I keep mollies in a 10-gallon?

No. Minimum is 20 gallons for a small group.

How many fry do they have?

Often 20–100+ every 4–6 weeks depending on female size and condition. Females store sperm and can drop multiple batches after one mating.

What should I feed them?

Veggie flakes or pellets as staple, algae wafers, blanched vegetables, and limited frozen protein. Let them graze between meals.

Are balloon mollies OK?

They sell well but the rounded body is a spinal deformity with real welfare costs. Prefer standard body shapes.

Related Resources

• Molly vs guppy comparison

• Platy vs molly

• 20-gallon stocking ideas

• Platy vs Molly comparison

• Guppy care · Platy care · Swordtail care

• Water testing and parameters

• Breeding aquarium fish

• Stocking guide · Aquarium tools

• Common setup mistakes