Mollies (Poecilia sphenops, P. latipinna, and related species) are among the most popular livebearers in the hobby, and for good reason. They come in a wide variety of colors and fin types — black mollies, dalmatian mollies, sailfin mollies, balloon mollies — and they breed readily in captivity.
However, mollies are often misunderstood. They are frequently marketed as beginner fish, but they are actually more demanding than guppies or platies when it comes to water chemistry. Mollies prefer hard, alkaline water with some mineral content, and many keepers report better health and longevity when salt is added — a nod to their natural brackish habitats.
Tank Setup
Tank Size
Mollies are larger than guppies (females can reach 3-4 inches, sailfin varieties up to 5 inches) and need more space. Minimum tank sizes:
- Short-finned mollies (common, black, dalmatian): 20 gallons minimum
- Sailfin mollies: 30 gallons minimum — they grow large and are active swimmers
- Balloon mollies: 20 gallons — though these are bred for a shortened body that can cause health issues
A 10-gallon tank is insufficient for mollies despite what some pet store employees suggest. Mollies produce significant waste for their size and need swimming space.
Water Parameters
| Parameter | Target Range |
|---|---|
| pH | 7.5-8.5 |
| Temperature | 75-82°F (24-28°C) |
| GH | 12-25 dGH |
| KH | 6-12 dKH |
| Ammonia | 0 ppm |
| Nitrite | 0 ppm |
| Nitrate | <30 ppm |
| Salt (optional) | 1-2 tbsp per 5 gal |
API Freshwater Master Test Kit
Essential Testing- ✓ Covers the critical parameters mollies are sensitive to
- ✓ Liquid reagents provide accurate readings
- ✓ 800+ tests make it cost-effective
- ✓ High Range pH test is important for mollies
- ✗ No GH/KH tests — buy separately for mollies
- ✗ Nitrate test requires vigorous shaking
Mollies are the one common livebearer that genuinely benefits from hard, alkaline water. While guppies and platies tolerate moderate hardness, mollies thrive in conditions that would be too hard for most tropical fish. If your tap water is soft (GH below 10), consider adding minerals or crushed coral.
The Brackish Question
Wild mollies are found in brackish environments — estuaries, mangrove swamps, and coastal marshes where freshwater meets saltwater. While they can survive in pure freshwater, many experienced keepers maintain that mollies show better health, fewer disease issues, and longer lifespans with added aquarium salt.
Adding 1-2 tablespoons of aquarium salt per 5 gallons creates a mildly brackish environment. This is not enough to harm freshwater plants or most community fish, but it can be incompatible with some tank mates — particularly neocaridina shrimp (pH 6.5-7.5, TDS 150-250) and caridina shrimp (pH 5.5-6.8), which do not tolerate salt.
Filtration
Mollies produce substantial waste. A strong HOB filter or canister filter rated for your tank size (or above) is recommended. Good biological filtration is essential — mollies are sensitive to ammonia and nitrite, and their heavy bioload can overwhelm undersized filters.
Feeding
Mollies are omnivores with a strong herbivorous tendency. In the wild, they graze on algae and plant matter throughout the day.
Hikari Fancy Guppy
Good Staple Food- ✓ High protein supports molly growth
- ✓ Spirulina content provides vegetable matter mollies need
- ✓ Semi-floating suits molly surface feeding behavior
- ✓ Affordable and widely available
- ✗ Pellets may be small for adult sailfin mollies
- ✗ Need supplementation with vegetables
Staple Diet
A quality flake or pellet with spirulina or algae content forms the base diet. Pure protein-heavy foods without vegetable content can cause digestive issues in mollies.
Supplemental Foods
- Blanched vegetables: Zucchini, spinach, peas (essential for digestive health)
- Spirulina wafers: Algae-based supplementation
- Brine shrimp or bloodworms: Protein treats 2-3 times per week
- Algae grazing: Mollies naturally graze on tank algae — beneficial to leave some growth
Feeding Schedule
Feed 2-3 times daily. Mollies are grazers by nature and do better with multiple small meals than one large feeding. Include vegetable matter in at least half of all feedings.
Breeding
Mollies breed prolifically — often more so than guppies. Females can store sperm and produce batches every 4-6 weeks without a male present after initial mating.
Gestation
Gestation lasts 60-70 days — longer than guppies or platies. Females become noticeably large and develop a squared-off belly profile before birth.
Batch Size
Mollies produce larger batches than most livebearers — 20-100 fry per batch depending on the female’s size and age. Sailfin molly females can produce 100+ fry in a single batch.
Fry Care
Molly fry are born larger than guppy fry and can eat crushed flake food and baby brine shrimp from day one. They grow quickly on a high-protein diet with frequent feedings (4-5 times daily).
Adult mollies will eat fry, though they are less aggressive about it than guppies. Dense plant cover (java moss, guppy grass, floating plants) dramatically improves fry survival in community tanks.
Common Health Issues
Shimmying (Molly Disease)
A distinctive symptom where mollies rock back and forth without moving forward. Caused by low mineral content, cold water, or poor water quality. Treatment: raise temperature, add aquarium salt (1 tbsp per 5 gallons), increase GH if below 12.
Ich
White spot disease is common in mollies, especially in tanks with unstable temperatures. Treatment: raise temperature to 82-84°F and add aquarium salt. Mollies tolerate salt-based ich treatments well.
Fin Rot
Ragged, deteriorating fins from bacterial infection. Usually triggered by poor water quality or aggression injuries. Treatment: water changes, aquarium salt, and antibiotics for severe cases.
Bloating
Mollies are prone to bloating from overfeeding or constipation. Prevention: include blanched peas in the diet regularly, avoid overfeeding protein-heavy foods. Treatment: fast for 24-48 hours, then feed blanched peas.
Tank Mates
Mollies work best with fish that tolerate hard, alkaline water:
- Platies: Similar care requirements, peaceful coexistence
- Swordtails: Compatible water parameters, similar size
- Corydoras: Bottom dwellers that tolerate moderate hardness
- Bristlenose plecos: Algae eaters that handle hard water
- Endlers/guppies: Compatible but may cross with endlers (rare)
Avoid: Soft water species (neon tetras, rams, discus), aggressive fish (cichlids other than peaceful species), and any shrimp if you add salt.
Frequently Asked Questions
Do mollies need salt in their water?
Not strictly, but many experienced keepers report better health with 1-2 tablespoons of aquarium salt per 5 gallons. Salt helps prevent shimmying disease and reduces susceptibility to fungal and bacterial infections. It is optional but beneficial for molly-only or compatible community tanks.
Can mollies live in a 10-gallon tank?
A 10-gallon is too small for most mollies. Short-finned varieties need 20+ gallons, and sailfin mollies need 30+. Mollies are larger and more active than commonly assumed, and overcrowding leads to health issues.
How many mollies should I keep together?
A minimum of 4-6, with a ratio of 2-3 females per male. Males harass females aggressively — multiple females spread the attention. A group of 6 in a 20-gallon tank is a good starting point.
Why do my mollies keep dying?
The most common causes are: soft/acidic water (mollies need hard, alkaline water), ammonia or nitrite exposure, cold temperatures, and overcrowding. Test parameters, ensure GH is above 12, pH above 7.5, and temperature 75-82°F.
Can mollies live with shrimp?
Adult cherry shrimp can coexist with mollies, but mollies will eat shrimplets. Adding salt (which benefits mollies) will harm shrimp. If you want to keep both, maintain a separate shrimp-only tank rather than combining them.
How long do mollies live?
Mollies typically live 3-5 years in proper conditions. Poor water quality, soft water, and overcrowding significantly shorten lifespan. Providing appropriate hardness and mineral content is the biggest factor in molly longevity.
Conclusion
Mollies are rewarding fish when their specific needs are met — hard, alkaline water (pH 7.5-8.5, GH 12-25), adequate tank size (20+ gallons), and a diet heavy in vegetable matter. They are not the easy beginner fish they are sometimes marketed as, but with appropriate conditions, they display beautiful coloration, breed readily, and live for several years.
If your tap water is naturally hard and alkaline, mollies are an excellent choice. If your water is soft and acidic, guppies, endlers, or platies may be easier starting points unless you are willing to buffer your water consistently.