Molly Fish Care Sheet

Molly Fish Overview

Mollies live in fresh and brackish (or slightly salty) waters throughout North, South, and Central America. This care sheet outlines basic care needs for a variety of molly fish types, including: 

• Black molly 

• Silver molly 

• Dalmatian molly 

• Gold dust molly

• Marble molly 

• Balloon molly 

• Lyretail molly 

• Sailfin molly 

Mollies come in an array of different colors and patterns, and their coloration may change or fade with stress or age. 

While some hobbyists recommend that mollies can adapt to living in a full saltwater aquarium over time, this isn’t recommended. Mollies are not native to saltwater environments and should only be kept in a freshwater or brackish aquarium. 

Molly Fish Characteristics 

Molly Fish Supply Checklist

To keep a molly happy and healthy, have these basic supplies on hand: 

• Appropriate-sized aquarium

• Appropriate food, dry and frozen

• Decor

• Water conditioner 

• Filter 

• Water test kit 

• Full-spectrum lighting 

• Net 

• Thermometer 

• Freshwater substrate 

• Heater 

• Airline tubing 

• Air pump 

• Check valve 

• Freshwater salt 

• Live plants 

• Aquarium hood/cover 

Molly Fish Tank Setup

For most species, up to two mollies can be housed in a 10-gallon tank, like the Aqueon LED Fish Aquarium Starter Kit, 10-gal.

If you decide to house a larger group of mollies, you’ll need to provide at least three gallons of extra tank space per fish beyond your first two. For example, a school of four mollies (which is also the recommended minimum school size) will need a 16-gallon aquarium or larger, like the Fluval Aquarium Kit, 16-gal.  

Bigger species, such as the sailfin molly, will need at least a 30-gallon aquarium (like the Coralife LED BioCube Aquarium Kit, 32-gal.) to swim and live comfortably.  

All aquariums must be secured with a fitted lid or canopy to prevent fish from escaping and getting injured. Always provide the largest habitat possible, as water parameters are less stable in smaller tanks. 

Selecting Tankmates 

Mollies should be kept in a group of at least four fish of the same species. They are naturally social, so if the aquarium is spacious enough, a larger group is even better! If you decide to keep a school of mollies, have one male for every three females in the tank to minimize aggressive behavior. 

Although mollies are naturally docile, they can become aggressive when housed in the same aquarium as aggressive fish. However, they can be kept in a community tank if it’s stocked with fish that have peaceful temperaments, including: 

• Platys 

• Guppies

• Swordtails  

• Tetras

• Gouramis 

• Rasbora 

• Rainbowfish 

• Danios 

• Cory catfish 

Gradually introduce new tankmates to an aquarium. Remember that the tank’s ammonia, pH, and nitrate levels will change when a new fish is introduced. Monitor these parameters carefully after adding new fish.   

Do not keep fish in overcrowded aquariums, as these conditions often lead to stress and disease in the tank. 

Filtration 

A filter system is essential for any aquarium. It keeps the tank clean, removes harmful toxins like ammonia, and adds oxygen to the water so fish can breathe.   

Choose a filter that provides slow to moderate water circulation, to mimic the currents found in mollies’ natural habitats. Power filters (also known as hang-on-back filters) and external canister filters are recommended because they offer effective mechanical, biological, and chemical filtration and do not take up any space inside an aquarium. 

Tip: Your filter should be powerful enough to process all the water in the tank at least four times an hour.

Water Health 

Test the aquarium’s water regularly to ensure its pH, ammonia, nitrate, and nitrite levels remain stable and safe. An aquarium test kit can quickly and accurately measure these key parameters.

Many molly species benefit from the addition of freshwater salt. If you use it, keep the water’s specific gravity at 1.004, with no more than a +/- 0.001 change in 24 hours.  

Research the needs of your specific species of molly before adding freshwater aquarium salt. Use a hydrometer or refractometer (salt level tester), like the Instant Ocean Testing Hydrometer for Aquariums, to accurately monitor salt concentration and specific gravity. 

While exact needs vary between species, most mollies thrive in water temperatures from 72 to 82 F. The temperature should not fluctuate up or down more than 2 degrees in 24 hours.  

Use an aquatic heater with a thermostat, like the Tetra HT30 Submersible Aquarium Heater & Electronic Thermostat, to keep the tank stable, and check the water daily with an aquatic thermometer. 

When selecting a heater for their tank, keep a few things in mind: 

• Some modern heaters have built-in thermostats, while others need to be paired with a thermostat that’s purchased separately. 

• As a rule of thumb, aquarium heaters need 2.5–5 watts of power for every gallon of water in a tank.

• Larger aquariums with a tank volume over 50 gallons may need two small heaters placed on opposite sides of the tank to prevent cold spots from developing in the water. 

Decor and Accessories 

Substrate

Line the bottom of a molly’s tank with 1 to 2 inches of freshwater sand or gravel. Always rinse the substrate with clean running water before adding it to the tank.   

• Mollies spend most of their time swimming near the top and middle of the tank rather than the bottom. 

• Sand typically anchors plants better than gravel. 

• Aquariums need about 1½ pounds of substrate for each gallon of water.

Plants, Rocks, and Hiding Decor

Mollies need plenty of hiding places to feel safe and secure. Both live and artificial plants work, though live plants also provide an extra food source. 

• Taller plants, like Java fern and anubias, are ideal. 

• Arrange plants around the tank’s perimeter to leave open swimming space. 

Cleaning and Maintenance

Perform routine water changes (no more than 10–25% of the aquarium’s total water volume) every two to four weeks. Remove uneaten food from an aquarium daily with the help of a fine mesh net. 

Avoid draining and replacing the aquarium's entire water volume, as doing so will remove the beneficial bacteria in the tank that keep the habitat’s ecosystem healthy.  

Most aquariums will need a full cleaning once a month, depending on the number of fish and invertebrates in the tank.

Molly Fish Food

Mollies are omnivorous and thrive on a varied diet. Offer a mix of:

• Flakes

• Pellets

• Freeze-dried and frozen foods (thawed before feeding)

• Small amounts of algae, such as spirulina, algae sheets, or algae wafers

To stay healthy, mollies need variety in their diet and should not be fed the same food every day. 

Feed one to three times per day (depending on size/species) and only as much as they can consume within one or two minutes.

Remember: Never use a microwave to thaw or warm frozen food, and never offer food that's still frozen. Discard any uneaten frozen food—do not refreeze it, as this can encourage bacteria growth. 

Molly Fish Veterinary Care

Annual Care

If your molly tank is the right size, is properly maintained, and you perform routine water quality tests and water changes, you’ll usually only need a veterinarian if a problem arises. Schedule an initial check of your setup with a veterinarian within a week of establishing the tank. Find an aquatics veterinarian who makes house calls, as transport can be very stressful for fish.

Signs of a healthy molly include:

• Clear eyes

• Full range of motion of fin movement

• Intact fins all the way to the edges with no discoloration

• Regular and active swim pattern

• Large appetite

• Swims at the top of the water column

Contact your veterinarian if you notice any of the following in your molly:

• Changes in color

• Lethargic swimming

• Receding fin edges with or without discoloration at the edges

• Decreased appetite for more than a day

• Itching

• Rapid breathing, sometimes with flared gills

• Gill color changes

• Presence of lumps, bumps, or masses

• White growths or spots of any kind

• Cloudy or bulging eye(s)

• Elevated scales

• Weight loss

• Bloating

Common molly fish illnesses include:

• Fin or tail rot

• Dropsy (fluid-filled coelomic cavity)

• Parasites

• Bacterial infections

• Cancer

• Ich

• Pop eye

• Fungal oral or skin infections

• Swim bladder disorders