Molly
Fish
The easygoing livebearer that shrugs at water most fish would sulk in!
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🤓 Did you know? Unlike most freshwater aquarium fish, mollies are naturally a brackish-water species that can tightly regulate their salt and water balance well past normal seawater concentration. Whether a healthy freshwater tank needs aquarium salt added is genuinely disputed among experienced keepers though, some use it routinely, others argue consistent water changes do the same job without salt's downsides for live plants and snails.
🗓️ The first week
The numbers a new owner needs in the first week, each from the article it links to. Rows with no link come from the encyclopedia.
- Tank size
- 20 gallons for a small group, 30 or more for sailfins or any group that has started breeding. Long rather than tall, with a lid, since mollies jump.Setup guide →
- Temperature
- 72 to 82°F, held steady: a swing of more than about 2°F in 24 hours is one of the commonly cited shimmy triggers. A heater unless the room stays warm year-round.Setup guide →
- Water chemistry
- pH 7.5 to 8.5 and medium-hard to hard water. Soft tap water gets crushed coral or a buffering product to bring it into range.Setup guide →
- Salt
- Not a standing requirement. Its place is shimmy treatment, 1 tablespoon of aquarium salt per 5 gallons in a hospital tank, and only plain aquarium salt, never marine mix or table salt.Setup guide →
- Filtration
- A hang-on-back or canister turning over the full volume at least four times an hour. A sponge filter for a breeding or nursery tank, so fry are not pulled into the current.Setup guide →
- Cycle first
- 2 to 4 weeks fishless, less with bottled bacteria, and a conditioner on every bucket of tap water. Ammonia and chlorine are both recognized shimmy triggers.Setup guide →
- Water changes
- 25 to 30% weekly from the substrate, with conditioned water at tank temperature. Test nitrate first: under 20 ppm the routine is right, past 40 ppm the next change is 50%.Aquarium Water Changes →
- Plants, a population lever
- Parents eat their fry. Dense live plants hide them and survival climbs; skip the cover and predation keeps the numbers down instead.Setup guide →
- Feeding schedule
- Adults once or twice a day, only what clears in a minute or two. Fry need small amounts at least 3 to 5 times a day.Feeding guide →
- Diet
- Tropical flake or pellet as the base with real vegetable content on top: blanched zucchini, spinach, cucumber, lettuce or peas two to three times a week. An all-protein diet is the most repeated molly mistake.Feeding guide →
- Sex ratio
- Two to three females per male, and more is better. An all-male group avoids fry entirely but needs space and numbers, since male mollies squabble more than male guppies.Enrichment guide →
- Fry are coming
- Gestation 4 to 6 weeks and broods of 20 to 60. Females store sperm, so store-bought females keep producing without a male present.Handling guide →
- Quarantine
- 2 to 3 weeks in a separate tank before new fish join the display. It catches most problems, shimmy included, before they spread.Health guide →
- Shimmy
- A side-to-side rocking as the fish swims, a sign of lost nerve and muscle control rather than a disease. Correct the water first: temperature toward 76 to 80°F, pH toward 7.5 to 8.5, hardness up if the water runs soft.Health guide →
- Budget
- $2 to $8 a fish for common short-fins, $15 to $20 for specialty strains. A 20-gallon setup runs $130 to $260 and upkeep $20 to $30 a month. Mollies rarely see a vet.Cost guide →
- Lifespan
- 3 to 5 years, up to 5 to 7 with stable water.Cost guide →
- Adult size
- 3 to 4.5 inches for short-fin varieties, 4 to 6 inches for sailfins.Handling guide →
- Power outage
- Oxygen is the threat, not darkness. Unplug the filter rather than letting it sit dead in the water, and keep the surface moving.Power outage guide →
🚨 Emergency card
Test the water first, then act on these today. Mollies rarely see a vet; nearly all of this is handled at home.
- A side-to-side, snake-like rocking as the fish swims: shimmy, fixed by correcting temperature, pH, and hardness
- Small white spots like grains of salt, with flashing against decor: ich, treated with copper sulfate or formalin medication plus the water fix that let it in
- A fine gold or rust-colored dusty coating, with appetite loss, lethargy, and scratching
- Ragged, frayed, or discolored fin edges
- Buoyancy problems: floating, sinking, or swimming at odd angles
- Scales standing out in a raised, pinecone pattern: dropsy, the hardest condition here to treat
Stress, unstable water, overcrowding, and skipped quarantine sit behind nearly every condition on this list.
📰 Where to go next
How Much Does Molly Fish Ownership Really Cost?
$2 to $8 a common molly, the $130 to $260 the tank costs around it, and the $20 to $30 a month after that.
Molly Tank Setup: A Real 20 Gallons, Sturdy Filtration, and the Salt Question
A real 20 gallons, 72 to 82°F, hard alkaline water, four-times turnover, and the salt question answered honestly.
Molly Feeding: Schedule, Diet, and Why Vegetables Matter
Once or twice a day, blanched vegetables two to three times a week, and why an all-protein diet is the molly-specific mistake.
Mollies Aren't Handled Either, But They Do Need a Breeding Plan
A netting-only fish, gonopodium versus gravid spot, and the breeding plan you need before you buy.
Common Molly Health Issues (Starting With the 'Shimmy')
The shimmy protocol, ich, velvet, fin rot and dropsy, and the quarantine window that stops most of it.
Molly Enrichment: The Biggest Livebearer Needs Room to Graze
The harassment research done on a molly, the sex ratio it implies, and why a spotless tank starves a grazer.
🛒 What to buy
Prices and the reasoning behind each item are in How Much Does Molly Fish Ownership Really Cost?.
❓ Frequently Asked Questions
What size tank does a molly need?
20 gallons is the minimum for a small group, noticeably more than the 10-gallon starting point that works for guppies. Sailfin varieties, or any growing colony, do better in 30 gallons or more. Go long and rectangular over tall, and use a lid, mollies can jump.
Do mollies need aquarium salt in their tank setup?
Not strictly, and this is more debated than most molly care advice lets on. Mollies are a real brackish-tolerant species biologically, but whether a healthy freshwater tank needs salt added is disputed among experienced keepers, some argue good water changes and stable parameters do the same job without the downsides salt carries for live plants and snails.
How do I manage molly population if I don't want constant breeding?
Keep males and females in separate tanks to avoid breeding entirely, or keep a males-only group for a colorful, breeding-free tank, which needs space and numbers rather than a trio because males squabble among themselves. If you do want controlled breeding, a ratio of roughly one male to two or three females cuts down on females getting chased and stressed by mating attempts.
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📰 Deep Dive
🐠 How Much Does Molly Fish Ownership Really Cost?
🐠 Mollies Aren't Handled Either, But They Do Need a Breeding Plan
🐠 Common Molly Health Issues (Starting With the 'Shimmy')
🐠 Molly Tank Setup: A Real 20 Gallons, Sturdy Filtration, and the Salt Question
🐠 Molly Feeding: Schedule, Diet, and Why Vegetables Matter
🐠 Molly Enrichment: The Biggest Livebearer Needs Room to Graze
🩺 Health and More
🐠 Aquarium Power Outages, Transport, and Moving Your Fish: The Complete Plan
🦠 How to Cycle a Fish Tank, Start to Finish: Fishless and Fish-In
🧽 Aquarium Filtration: How It Actually Works and How to Maintain It
🪣 Aquarium Water Changes: How Much, How Often, and What the Number Is For
🐟 Aquarium Stocking and Tankmates: How Many Fish, and Which Ones Go Together
🧪 Freshwater pH, GH, and KH: Why Chasing a Number Backfires
🤩 Fun Facts
🐠 At Home in Two Kinds of Water
Mollies are "euryhaline," meaning they can tolerate an unusually wide range of salinities for an aquarium fish. Wild populations turn up in everything from coastal saltwater lagoons to freshwater roadside ditches, sometimes just a short distance apart.