How many Molly (Common / Sailfin) in a 29 gallon tank?

A 29-gallon (110 L) tank comfortably holds about 4–6 Molly (Common / Sailfin) as a single-species display — a conservative range that leaves room for tankmates and a margin for error.

Molly (Common / Sailfin) key facts

TankStocking sizes this from a per-species footprint — the territory and waste a single Molly (Common / Sailfin) needs in a comfortable display tank — rather than the unreliable "inch per gallon" rule. On that basis a 29-gallon tank comfortably suits about 4–6 Molly (Common / Sailfin). Keep a minimum of 3 — Molly (Common / Sailfin) is a social species that suffers in smaller groups. This is a comfortable display range, not a hard ceiling: stocking lighter keeps water quality stable and leaves headroom for tankmates and growth.

They get along with most peaceful fish but are larger and messier than guppies, so they need more space and stronger filtration. The engine clears the usual community crowd; plan for breeding and for their preference for harder water (some keepers add a little salt, which limits invertebrate tankmates).

Want a community rather than a single-species tank? The Molly (Common / Sailfin) tank mates guide lists the species the engine clears with Molly (Common / Sailfin).

Keeping Molly (Common / Sailfin) well: the detail

Beyond the table — the behaviour, the parameters that actually move the needle, and the mistakes that cost fish. Hobbyist context, not a verdict; the planner above is what judges your plan.

Water parameters in practice

Mollies are the hardest-water fish on this list: they need genuinely hard, alkaline water (GH 15–30 dGH, pH 7.5–8.5) and visibly struggle in soft, acidic tanks. Do not soften the water for them — if anything, harder is better. They are large and messy, so strong filtration and a roomy tank are part of the parameter set, not extras.

Why Molly (Common / Sailfin) behaves the way it does

Mollies come from hard, mineral-rich coastal and brackish waters of Central America, where some populations live in genuinely salty estuaries. That ancestry is why they crave hardness, tolerate a little salt, and carry a heavier build and bio-load than a guppy or platy. They are active, social grazers that browse algae and biofilm all day.

Common Molly (Common / Sailfin) stocking mistakes

The first mistake is keeping mollies in soft water, which quietly stresses them and invites disease. The second is underestimating their size and waste — common mollies want 20 gallons and sailfins 30-plus, with filtration to match. The third is reflexively adding salt: mollies do not need it in freshwater, and it rules out shrimp and many catfish.

Early signs something is wrong

The molly tell-tale is "the shimmies" — a fish that shudders and swims on the spot without going anywhere, almost always a sign of soft water, cold, or poor quality. Clamped fins, gasping at the surface, and white patches follow from the same neglect. A thriving molly grazes constantly and holds its dorsal high.

How tank size changes the answer

Common mollies need at least 20 gallons; sailfins, which reach a hefty 12–15 cm, want 30 or more for their size and their waste. The extra volume is about bio-load as much as swimming room — crowd mollies and water quality collapses fast. A mixed group breeds readily, so plan for fry or keep one sex.

Stocking in stages: cycle first, then add slowly

The stocking numbers on this page assume one thing the calculator can't do for you: a cycled tank, filled gradually. Even with the volume to spare, adding a full stock at once can outrun the filter bacteria — stage it.

1. Cycle before any fish. Run a fishless cycle — an ammonia source plus the filter — until the tank processes ammonia and nitrite to zero within 24 hours. That typically takes four to six weeks. This is the step that prevents most early losses.
2. Add one group at a time. Introduce a single species or group, then wait a fortnight before the next, testing the water in between. Each addition is a load the filter bacteria have to catch up with.
3. Heaviest waste producers last. Bring in the big, messy fish once the tank is mature and the bacteria are well established, not on day one.
4. Stop short of the limit. The comfortable numbers above leave headroom on purpose. Stocking lighter keeps water quality stable and gives you room to correct mistakes.

Test your real, staged plan in the planner above — it scores the full list against the same welfare engine, whatever order you intend to add fish in.

Common questions

More on stocking Molly (Common / Sailfin)

Related guides on TankStocking — each scored by the same welfare engine as the planner.

Stocking numbers are planning estimates — filtration, planting, and maintenance all shift the real ceiling. Cycle the tank and stock gradually. How this estimate works →