Black molly

Poecilia sphenops

FreshwaterBrackishFish

Care at a glance

Temperature: 70–82°F (21–28°C)
pH: 7.0–8.0
Adult size: 4.7 in (12 cm)
Minimum tank: 24 gal (90 L)
Temperament: Mostly peaceful
Social behavior: Social
Swim level: Middle
Difficulty: Easy
Breeding: Easy
Rarity: Very common
Origin: Venezuela, Colombia, Mexico
Family: Poeciliidae

Overview

Mollies are social and mostly peaceful, so keep a small group, ideally with more females than males to spread out the males' constant chasing. They swim in the middle of the tank and like a 90 liter or larger setup with hard, alkaline water, 21 to 28 C and pH 7 to 8. They graze algae and need real plant matter, so feed a vegetable-based flake or pellet with some protein mixed in. They breed easily, so expect fry.

In the wild

Black mollies trace back to coastal fresh and brackish waters in Mexico, Colombia, and Venezuela, including streams and estuaries near the sea. Because of that brackish background they tolerate, and often appreciate, hard, alkaline water with a little salt. A planted tank with open swimming space and steady warmth suits the all-black aquarium strain well.

Good to know

The black molly is a melanistic, captive-bred color form of Poecilia sphenops, in the livebearer family Poeciliidae. It grows to about 12 cm, with females larger and rounder than males. Like other livebearers, it gives birth to free-swimming young instead of laying eggs. Males are identified by the gonopodium, a modified anal fin used to fertilize females.

Did you know

Black mollies are prone to a swimming quirk called the shimmies, where the fish wiggles in place without going anywhere. It's not a disease itself but a stress signal, usually from water that's too soft, too cool, or has gone off, and it often clears up once conditions improve.