Brackish Water Livebearers

Mollies

Mollies are the largest commonly sold livebearers, growing to an adult size of around 8 to 15 cm (4 to 6 inches). They are usually put in the same genus as guppies, Poecilia, but other authorities consider them to be distinct enough to deserve a separate genus, Mollienesia, from which the name “molly” is derived.

In the wild mollies are found in a wide range of waters -- from freshwater streams through to the open sea -- and to some extent even populations within species often have quite specific water requirements. This is only really an issue with wild caught mollies though (in which case ask your retailer what conditions the fish are being kept in); standard issue captive bred stock can be assumed to be hardy and adaptable. Hard, alkaline water suits captive bred mollies best, and the two sailfin mollies as well as the black molly all thrive in brackish conditions. Black mollies are commonly used to 'run in' marine aquaria, requiring little more than a few water changes across 30-60 minutes to be safely adjusted between freshwater and marine conditions.

Mollies appreciate warmth, 25 to 29-degrees Celsius being about right; but ensure aeration of the water is adequate since warm water holds less oxygen. They also need good filtration and a strong current. Mollies are very largely vegetarian, and do best on a diet consisting of algae, vegetarian flake foods, and small invertebrates such as bloodworms and Daphnia. Blanched lettuce, spinach, and cucumber can be placed in the tank as well, and the fish will peck at them over the following days quite happily. Mollies need space if they are to look their best, and the sailfin species in particular are potentially large fish and need to be quartered accordingly. They are essentially peaceful fish and should never be kept with aggressive fishes. However, male mollies can be very aggressive towards one another, especially the sailfin varieties. Males are also rather pushy towards the females, so it is always a good idea to keep more females than males.

While mollies spawn easily, actually raising good quality stock is difficult. They need excellent water quality and a high pH and hardness to grow fast. The fry need plenty of space if they are to grow to their maximum size, and male sailfin mollies are especially prone to being stunted if kept in inadequate quarters.

Sailfin mollies

There are two sailfin mollies traded. The common sailfin molly Poecilia latipinna is most widely sold in a variety of artificial colour morphs including red, orange, black, and white. The natural form is a silvery green with some orange and brown markings on the sides and fins. Only the male has the sail-like dorsal fin. Females reach around 8 to 10 cm (3 to 4 inches) in length, while the males are a bit smaller.

The giant sailfin Poecilia velifera is not so commonly traded. Male giant sailfin mollies typically reach around 8 to 10 cm (3 to 4 inches) in length, but female giant sailfins can potentially top 15 cm (6 inches).

Commercial strains of both sailfin mollies do best in slightly brackish water with a specific gravity of at least 1.001, and they can be adapted to moderately brackish water, up to 1.010, without much trouble. Giant sailfin mollies in particular enjoy strongly brackish water and will thrive in marine conditions as well.

Shortfin mollies

Like the common sailfin molly, shortfin mollies are most often seen in a variety of artificial colours as well as forms that have been manipulated into aberrant shapes including balloon mollies and veil-tail mollies. There are several species of shortfin molly, but the artificial forms are usually hybrids of Poecilia sphenops and Poecilia mexicana. 

Obtaining wild-caught or even pure-bred strains of these species is very difficult. Shortfin mollies reach about 8 cm (3 inches) in length depending on the species.

Several shortfin molly species actually do best in fresh rather than brackish water, but the commercially available stock will do fine in slightly to moderately brackish water (SG 1.001 to 1.010).

Black mollies

Black mollies are artificial hybrids of various molly species. Although usually kept (wit varying degrees of success) as freshwater fish, they do very well in brackish or salt water. Because of their velvety black colouration, they are very popular in community tanks, but they also make a very fine contrast to the greens of a densely planted aquarium as well.