Mollies are lovely additions to your home aquarium. They’re eager to reproduce which has led to them being available in a wide range of unique colors and fin shapes over the years.
They’re easy to care for too, which makes them excellent pets for the new or practiced aquarist.
You can find them in the wild from the Southeast United States to the Yucatan Peninsula in Mexico. Different Molly Fish species live in various salinities, including freshwater, brackish water, and on rare occasions, saltwater.
They have the unique ability to adjust to higher salinities, although almost all in the aquarium trade prefer saltwater.
All About Molly Fish
Behavior & Temperament
They’re generally nonaggressive and most fish stores will have them labeled as such. However, I have seen Molly Fish pick on a Platy or Guppy occasionally.
When keeping a shoal of Mollies, make sure to have a minimum of three fish: two females and a male. Females are distinguishable by their rounded anal fin and large round middle body. Males have more narrow bodies and large sweeping anal fins.
Males are more aggressive, particularly towards females, so keeping multiple females discourages them from bullying one. An ideal number of mollies would be five but would require a larger sized aquarium.
Mollies do not swim at the top or bottom but prefer to school in the middle of the tank. Mollies will school, but not as often as other schooling fish.
Mollies are omnivores, meaning they eat both meat and plants. Like most animals feeding a varied diet will increase the lifespan of your fish. In their natural habitat, Molly Fish are more likely to eat plants but they’re not picky eaters. Ideal foods include:
- Algae Wafers
- Blanched Veggies (Spinach)
Try to keep the majority of their diet as plants.
Mollies can be fed up to twice a day and should not take longer than three minutes to eat their food. New fishkeepers tend to be surprised by the small amount of food needed to feed their fish. They are susceptible to bloat, though, so overfeeding can be deadly in this species of fish.
Be careful not to overfeed and check for excess nitrates as a sign of an overindulgent diet.
How long do they live?
With proper care, Mollies live on average five years.
Molly Fish are beautiful and come in a wide range of colors and patterns. Female Mollies are traditionally larger and can reach 4.5 inches, males usually only reach 3.5 inches. I have seen specimens as large as 5 inches, though it isn’t common.
Mollies have a wide variety of appearances. Most have a triangular-shaped head with a slim or rounded body and a small rounded caudal tail. Due to this species being “live-bearers,” the differences between a male and female are pronounced.
- Male Mollies have a sharp thin anal fin, females have a rounded anal fin.
- Females have a much more rounded stomach, while the male has a slimmer shape.
- Male Sailfin Mollies have a sizable (and spectacular) fin on top of their head, hence their name.
Female mollies have a spot above their anal fin called a grandid spot. This dark spot is due to the stretching caused by pregnancy, the whole area above their anal fin is typically darkened if the female has had babies before.
Mollies have many different species, all with unique looks; some “common” Molly Fish include
- Short-finned Molly
- White Molly
- Black Velvet Molly
- Balloon Molly
- Dalmation Molly
- Lyretail Molly
- Common Molly
- Golden Molly
Setting up your Molly Fish Tank
Technically, you can keep Molly Fish in a ten-gallon aquarium, although I would suggest a minimum of 20 gallons.
If you begin to have issues with aggressive Mollies, consider the size of your aquarium. If your Mollies are around four inches and have five or more other fish in the tank, they are more likely to be stressed and exhibit aggressive behavior.
Type of Substrate
Mollies do not have a particular need when it comes to the substrate. If you would like to plant the tank, consider substrate preferable for planting your tank, such as sand or a substrate that offers your plants nutrition. Your Molly will rarely go to the bottom of the tank and should not have contact with your substrate.
Mollies do not require heavy filtration. Their oxygen needs are on the lower end, despite their near-constant movement. Consider live plants to help aid oxygen, and either a hang over the back filter to accommodate the size of your aquarium or a sponge filter set up with an air pump.
Make sure to clean out the filtration about once a month; most filtration pads, sponges, etc., suggest every three to six weeks. Use your better judgment. Do not allow the filtration to get gross, but do not feel like you need to replace a perfectly clean filter pad.
When cleaning a sponge, be careful not to clean it with chemicals; you can soak in algae fix or simply give the sponge a decent rinse with water treated for chlorine and chloramines. Do not wash your filter and do a water change on the same day. When you do this, you risk hurting an established biological filter, especially for smaller aquariums.
Mollies have no specific needs when it comes to lighting. Like most freshwater tanks, your water quality and quality of life of your fish will improve with the addition of plants. If you plan to plant your tank, look for plant-specific lighting that offers a spectrum of colors needed for better plant growth.
If you feel the lighting is too intense, add plants such as hornwort or pennywort to help shade the rest of the tank. This will give you the bonus of less stressed fish and a reduction in algae growth.
Plants and Decorations
Like most fish species, Molly Fish prefer small caves and rock pieces, similar to arrangements found in their natural habitat. The more small caves and “hiding” areas you have for your Mollies, the more relaxed your fish will be.
When choosing different decorations, be wary of wood pieces. Although not bad for the tank inherently, tannins tend to soften the water and Molly Fish prefer hard water. If you decide to get a wood piece, make sure to rinse well and cure the wood to avoid leeching tannins. If you want to skip this process altogether, buy fake wood pieces for your aquarium.
Plants are an excellent addition to most freshwater aquariums. Plants reduce nitrates and phosphates in your water and help to create more oxygen. Plants not only improve the water quality but will help to control algae growth in your aquarium.
Plants that shoot up, such as pennywort or lilies, work well to help shade your aquarium and create a more natural environment for your fish. Low-growing plants are ideal as well. If you get plants that cover more of the middle to the tank, just make sure you still have plenty of room for your Molly Fish to swim around comfortably.
Any other common aquarium decorations will likely be safe. When choosing decorations, choose aquarium-safe decorations, or make sure to read over all the plastics and dyes in your decorations to see if they are aquarium safe.
Although I have seen Molly Fish nipping other fish, they are listed as a non-aggressive species. If you have a Sailfin Molly species, be extra careful to watch for tankmates nipping on fins due to their sensitivity.
Ideal tankmates include:
- Dwarf Gouramis
- Smaller or similar-sized Tetras
- Cory Catfish
- Non-aggressive Loach species
Tank Environment for Molly Fish
Mollies do best at a temperature from 72-78 degrees Fahrenheit. They will tolerate warmer and colder waters but they’re healthiest in this range.
Molly Fish do not require a high water flow. If you have a smaller tank and would not like to keep an over the back filter, feel free to use a simple sponge filter or two attached to the air pump.
Hardness Levels & pH
Like most fish, Mollies prefer a stable pH. Although they can tolerate a high pH range, if the pH goes up or down too quickly, the fish are more susceptible to disease or can even die from shock.
An ideal pH for Molly Fish is between 6.7 to 8.5, a relatively high pH for a freshwater fish. If you have issues keeping your pH high, carefully add pH Up or an equivalent from a pet store to your water.
These fish are used to hard water. Their hardness should be between 15-30 dGH; this is easy to keep up with using treated tap water since tap water is usually harder. If you’re still having trouble raising your hardness, you can add limestone or crushed coral to your aquarium. Do this very slowly to be careful not to elevate the pH too quickly.
Chlorine and Chloramines
When you use RO (reverse osmosis) or DI (deionized) water, you are less likely to run into contaminants or chlorine/chloramines. Chlorine is toxic to fish when introduced into their aquarium water. It’s best to test regularly for chlorine or chloramines, but if your fish is lethargic or has white spots, you can test for chlorine in your water supply.
The easiest way to treat for chlorine in your aquarium water is to use a good water conditioner. Make sure to use the conditioner each time you do a water change to prevent introducing chlorine to your water supply.
Ammonia, Nitrite, and Nitrate
Very few fish will tolerate any amounts of ammonia or nitrite. Even the fish species that CAN tolerate them do not survive for long and are more prone to ailments. That’s why it’s so essential to cycle an aquarium before the introduction of fish.
To start a cycle, use aquarium starter bacteria or liquid ammonia. Decide which one is best for you, and adequately dose your aquarium before introducing fish. The tank should be ready to go in all other ways, including lights, substrate, filtrations, etc.
Most bottles will say fish are safe to add after 24 hours. This is an optimistic assumption. It is safer to add fish after tracking with water quality tests that both ammonia and nitrite have previously been detected but are no longer present.
If you decide to add your Molly Fish and notice an addition of ammonia or nitrite in your water quality, add more liquid bacteria and do a small water change (less than 25%).
To ensure a proper cycle is maintained, make sure to:
- Add fish slowly (do not add the full capacity of fish to your tank all at once).
- Watch water quality and act accordingly.
- Perform weekly water changes, and be careful not to clean filters the same day you perform water changes. Ideally, water changes are not over 25% of the tank water unless in severe circumstances.
- Do not overfeed your fish or leave uneaten food in your aquarium.
Ideal parameters or ammonia, nitrite, and nitrate are:
- Ammonia: 0ppm
- Nitrite: 0ppm
- Nitrate: less than 20ppm
Molly Fish are popular due to their fun colors and patterns and their tendency to reproduce. They’re known as live-bearers, meaning unlike most fish, they birth live-young rather than eggs.
They breed quickly, given that water conditions are ideal, and the temperature is warmer (78 degrees or so, but not higher). Make sure, as stated earlier, that there are more females kept than males, so the males do not harass one female and stress her out.
Males have a courting behavior but will also “sneak attack” females to copulate. If you notice copulation behavior and/or one of your females gets a bit larger, there’s a good chance you’ll have some babies in your tank soon. You can move mom to a breeder net or breeder tank to prevent the babies from being eaten by their parents once they’re born. If not, there’s a good chance you’ll lose many or all of them.
The breeder tank does not need elaborate accommodations but ideally would include a sponge filter instead of a traditional filter that might trap fry.
On average, 35-45 days after copulation, the mother will have up to 100 fry. The fry will look like smaller versions of their parents. You can feed them small pellets, broken pieces, or powdered spirulina.
The mother does not care for her young; the fry can be introduced to the main aquarium when they are too large to eat.
Interestingly enough, mother fish and the fry possibly have a symbiotic relationship where there is an exchange of fluids between the fry and the mom. This is not well understood and is mainly speculative.
Molly Disease is also known as livebearer disease or shimmies. Although not a disease per se, this ailment causes the fish to wiggle or swim erratically. The fish acts this way as a reaction to stress. To improve their condition, check your water quality and address any issues. If you do not recognize any problems with the water quality parameters, consider their tankmates or other environmental conditions as possible stressors for your fish.
Ich is a common freshwater parasite that appears when fish are stressed. Ich will display as white spots covering the body of your fish, particularly eyes and gills. If you recognize ich on one of your fish, remove them and place them in a quarantine aquarium immediately.
There are many different methods for curing ich. If only one fish displays ich, observe the others and monitor the water quality for any issues. Often ich presents itself when there are water quality issues, or a fish is particularly stressed.
Any parasite remover or herbtana can help treat the parasite. Dose appropriately and offer plenty of oxygen. Many aquarists suggest saltwater baths, but I disagree due to the additional stress undergone by the fish.
Velvet is similar to ich. It is a parasite that can affect most fish types. Fortunately, many fish have a natural immunity to it. Velvet will look similar to ich but it has a golden sheen to the dots found around their body.
Remove any infected fish to a quarantine aquarium. Raise the temperature a couple of degrees and add aquarium salt. Make sure no carbon or plants are present. Only put on lights for a couple of hours every day and use a copper treatment such as copper sulfate. Dose appropriately according to the instructions on your bottle. Copper treatments are ordinarily available at fish stores or online.
Certain fish die when they come in contact with copper. Be careful to check that copper treatments are safe if there are other fish or invertebrates are in your aquarium. Most inverts (like shrimp) will die if their water is treated with copper.
There are a couple of different types of bloat. The one I’ve seen most commonly in Mollies involves an inability to control their swimming or a sort of trouble staying either up or down in the tank. A swollen belly usually accompanies this. There are many possible reasons for this behavior.
Treat the tank for bacterial and fungal infections. You can typically alternate these medications every other day. Do not offer any food to your fish for three days and then offer one unshelled pea at the end of the third day.
Sometimes bloat is a side-effect of overeating. It might seem like a bad idea to stop feeding your fish, but fish can go much longer than three days without food.
Bloat is often hard to treat and often results in the loss of your fish.
So, now you should know everything you need to know to properly care for your Molly fish. Which is a good thing, because a happy and healthy tank of Mollies is a true delight. Plus, you get to see babies be born and grow up.
I hope you’ve found this care guide helpful! Good luck and happy fishkeeping.