Molly
Poecilia sphenops
Also known as Common Molly, Black Molly
Edited by Ivan F. · Reviewed
Beginner verdict
A peaceful, algae-grazing livebearer that needs hard, alkaline water to stay healthy. That water need matters more than anything else, and it is why mollies are not suited to a general soft-water community. In hard water they thrive, graze constantly, and breed with very little encouragement.
Care at a glance
- Min tank size
- 21 US gal
- Footprint
- 35 x 12 in
- Temperature
- 70–82 °F
- pH
- 7–8.5
- Hardness
- 15–30 dGH
- Swim zone
- Top
- Min group
- 3 or more
- Lifespan
- 3–5 years
Molly tank mates and swim zone
Top
Spends most of its time in this part of the tank.
Hemigrammus erythrozonus
Danio rerio
Puntius titteya
Barbodes semifasciolatus
Corydoras aeneus
Corydoras aeneus (albino)
Danio margaritatus
Molly is much larger than Celestial Pearl Danio and may frighten or even eat it.
Tanichthys albonubes
Their temperature ranges barely overlap, so you will have little room for error.
Poecilia wingei
Endler's Livebearer is small and shy, and is easily outcompeted for food by Molly.
Betta splendens
Betta often does best alone. Only try this in a larger, planted tank, and watch closely.
Hyphessobrycon herbertaxelrodi
One needs much harder water than the other can tolerate.
Common tank-mate questions
Molly feeding and care
How to feed
- Feed flakes with plenty of vegetable content, because mollies graze on algae.
- Add blanched vegetables or frozen foods for variety.
- Feed small portions you can see finished within a couple of minutes, because overfeeding fouls the water fast.
Telling males from females
Males are smaller and more colorful, with a pointed anal fin called a gonopodium. Females are larger and rounder.
Molly care mistakes to avoid
- Do not buy just one or two. Keep at least 3 fish so they feel secure.
- Only keep them in hard, alkaline water. Soft water leaves mollies prone to illness.
- Check tank mates share that hard water, as many tetras and rasboras do not.
- Expect fry. Keep more females than males to spread out the chasing.
- Check your tank before you buy. Our sourced minimum is 21 US gallons.
Molly tank setup
A forgiving setup with room for a small community.
TankAt least 21 US gal (81 L), 90 x 30 cm
The smallest tank our sources give for Molly. We list only tanks that hold this volume (allowing 10%, so a standard size counts) and have at least this much floor.
Picks for Spain are still being checked. The options below are for the US, and we have not confirmed that they are sold in Spain.
Aqueon Standard Glass Aquarium 40 Breeder US
Why we chose these (Best 20-Gallon Fish Tank: 20 Long or 29?). Nobody here has used or tested them.
Starter kitA kit with the tank, filter and light in one box
We list only kits whose tank fits Molly, up to twice its minimum volume. A kit whose heater has a fixed setting shows only when that setting is inside Molly's temperature range.
None of the kits we recommend fits Molly yet. See Best Aquarium Starter Kit: What to Look For for what to look for.
FilterHang-on-back or large sponge filter
Easy to maintain and strong enough for a community without blasting the fish.
Seachem Tidal 55 Power Filter US
Marineland Penguin PRO 175 Power Filter US
Eheim LiBERTY 130 Hang-on Filter Spain
Seachem Tidal 35 Power Filter Spain
Why we chose these (Best Filter for a 20 to 29-Gallon Tank). Nobody here has used or tested them.
HeaterKeep it at 70–82 °F (21–28 °C)
Adjustable heater, about 5 W per gallon: 100 W at 20 gallons, about 150 W at 29. Enough power to hold a stable temperature in most homes. Size up for a cold room.
Aqueon Pro Adjustable Heater 100 W US
Aqueon Submersible Glass Heater 100 W US
Eheim Thermocontrol e 100 Heater Spain
Tetra HT 100 Electronic Heater Spain
Why we chose these (Best Heater for a 20 to 29-Gallon Tank). Nobody here has used or tested them.
Water conditionerDechlorinator such as Seachem Prime
Tap water contains chlorine or chloramine that harms fish.
Seachem Prime US
Fritz Complete Water Conditioner US
API Betta Water Conditioner US
sera aquatan Spain
Tetra AquaSafe Spain
Sold at Kiwoko · Sold at Tiendanimal · Sold at zooplus.es · Maker's details
Why we chose these (Best Water Conditioner for Betta Fish). Nobody here has used or tested them.
Test kitLiquid kit such as API Freshwater Master
Tracks ammonia, nitrite, nitrate and pH while the tank cycles.
API Freshwater Master Test Kit US
API General & Carbonate Hardness Test Kit US
See it at The Shrimp Farm (a retailer; affiliate link) · Sold at Bulk Reef Supply · Maker's details
sera aqua-test box (Cl) Spain
JBL PROAQUATEST COMBISET Plus NH4 Spain
Why we chose these (Best Test Kit for a Freshwater Aquarium). Nobody here has used or tested them.
SubstrateAny substrate
We are still choosing products for this.
Hiding places and decorA planted tank is preferable.
We are still choosing products for this.
PlantsMostly safe with live plants
Cycle the tank before adding animals. See our guide on how to cycle a fish tank.
Where to buy Molly
We have not found a store that sells Molly under its scientific name yet. We add stores as we confirm them.
Live animals and plants travel by courier. Check the store's live-arrival guarantee, and only buy once your tank has cycled (see how to cycle a fish tank).
Some US states restrict certain species. Check our US state aquarium rules table and your state's own rules before you buy.
Molly FAQ
What size tank do Mollies need?
Our sourced minimum is 21 US gallons (81 liters), with a footprint of 90 x 30 cm. A bigger tank is always easier to keep stable.
How many Mollies should I keep?
Keep at least 3 fish. Bigger groups usually look better and behave more confidently.
What can live with Mollies?
Good matches on our list include Glowlight Tetra, Zebra Danio and Cherry Barb. Avoid Neon Tetra and Cardinal Tetra.
How long do Mollies live?
Care guides report about 3–5 years. Stable water and a good diet make the biggest difference.
Do mollies need salt?
They need hard, alkaline water far more than salt. If your tap water is soft, consider a different species rather than trying to adjust your water.
Molly facts and sources
- Family
- Poeciliidae
- Native to
- Mexico to Colombia
- Adult size
- 3.1 in (8 cm)
- Temperament
- Peaceful
- Social needs
- Harem/Pairs
- Diet
- Omnivore
- Water flow
- Gentle
- Substrate
- Any
- Live plants
- Mostly Safe
- Shrimp
- Adult Shrimp Only
- Fin nipping
- None
- Vulnerable to nippers
- No
- Breeding
- Easy Livebearer
Sources
- Seriously Fish: Tank size, water ranges and behavior
- FishBase: Cross-check of water ranges and size
Lifespans are approximate and come from aquarium care guides, because these databases rarely publish them.
Where sources differ, we show the Seriously Fish aquarium figures.
Spotted something wrong? Report a parameter correction. For a sick fish, see our veterinary disclaimer.