| Temperature | 24–28 °C (75–82 °F) |
|---|---|
| pH | 7.5–8.5 |
| KH | 3–10 dKH |
| GH | 4–12 dGH |
| Min tank | 20 gal (76 L) |
| Adult size | 8 cm |
| Lifespan | 3–5 years |
| Diet | Omnivore |
| Temperament | Peaceful |
| Origin | Mexico & Central America |
| Good tank mates | platies, guppies, snails, corydoras |
| Avoid with | soft-water tetras, aggressive cichlids |
Mollies are robust livebearers that actually prefer the hard, alkaline water many community tanks already have. The black molly is a classic aquarium fish.
At a glance
The molly (Poecilia sphenops) is a livebearer that thrives in harder, more alkaline water than most fish. Native to Mexico & Central America, adults reach about 8 cm and live 3–5 years with stable care.
Tank size and water
A 20-gallon tank is the practical minimum for a small group. Target 24–28 °C and a pH around 7.5–8.5. They are fine singly but look best in a small group. Like all fish they need a fully cycled tank with ammonia and nitrite at zero.
Pro tip
Mollies benefit from a little aquarium salt and vegetable matter. They are prone to stringy white poop if diet lacks greens.
Substrate and aquascape
Open swimming room with some plants. They tolerate brackish conditions better than most freshwater fish.
Diet
Omnivores with a strong vegetarian side: algae wafers, spirulina flakes, and blanched veggies alongside regular flakes.
Tank mates
Good companions include platies, guppies, snails, corydoras. Avoid soft-water tetras, aggressive cichlids. Temperament is peaceful, so match them with other peaceful community fish.
Breeding and behaviour
Livebearers that breed readily; females are noticeably rounder than males.
Common health issues
Most problems come from unstable water rather than disease. Read how to cycle a tank and watch for gasping at the surface, white spot, and fin rot so you can act early.
How to recognise a healthy Molly
Before you buy, watch the tank at the shop for ten minutes. A healthy Molly (Poecilia sphenops) is alert and upright, holds its colour, and eats readily. Fins should be intact and unclamped, the belly lightly rounded rather than hollow, and the skin free of white speckling, cottony patches or red streaks. Avoid any fish that sits at the bottom, rubs against decor, or hangs at the surface gasping — those are early signs you may meet common fish diseases soon after purchase.
Tank setup that suits Molly
In the wild this fish comes from Mexico & Central America. Mirror that at home with a 20-gallon-or-larger tank, a soft sand or fine-gravel substrate, and plenty of live plants or driftwood for cover and territory. Stable warmth around 24–28 °C (75–82 °F) and a gentle but effective filter keep stress low. They do fine singly but often look their best in a small group.
Water parameters
Target 24–28 °C (75–82 °F) and a pH near 7.5–8.5. The single most important rule is stability: a cycled tank with ammonia and nitrite at zero matters more than hitting an exact number. Test weekly, and do regular partial water changes to keep nitrates under control.
Feeding Molly
They are omnivore by nature, so offer a varied diet: a quality dry staple plus frozen or live foods a few times a week. Feed only what they clear in about a minute, once or twice daily for adults. Overfeeding is the fastest route to poor water quality and ammonia spikes, so when in doubt, feed a little less.
Tank mates
Good companions for Molly include platies, guppies, snails, corydoras. Avoid housing them with soft-water tetras, aggressive cichlids. Always match tank mates by temperament and adult size, not just by how they look in the shop.
Common problems to watch for
Even well-kept Molly can run into trouble. The most frequent issues are parasitic and bacterial infections, poor water quality, and stress from overcrowding. Quarantine new arrivals in a separate quarantine tank for two to four weeks before they join the main display.
A simple weekly routine
Keep Molly thriving with a repeatable habit: test water weekly, top up with matched-temperature water during a partial change, wipe algae from the glass, and glance at the filter flow. A calm, consistent routine prevents most problems before they start.
How to recognise a healthy Molly
Before you buy, watch the tank at the shop for ten minutes. A healthy Molly (Poecilia sphenops) is alert and upright, holds its colour, and eats readily the moment food hits the water. Fins should be intact and unclamped, the belly lightly rounded rather than hollow, and the skin free of white speckling, cottony patches or red streaks. Eyes should be clear, not clouded. Avoid any fish that sits on the bottom, rubs against decor, or hangs at the surface gasping — those are early signs you may meet common fish diseases soon after purchase. Buying healthy stock is cheaper than any medicine.
Tank setup that suits Molly
In the wild this fish comes from Mexico & Central America. Mirror that at home with a 20-gallon-or-larger tank, a soft sand or fine-gravel substrate, and plenty of live plants or driftwood for cover and territory. Stable warmth around 24–28 °C (75–82 °F) and a gentle but effective filter keep stress low, and stress is the root cause of most illness. They do fine singly, but often look their best and act most confidently in a small group. Good planting also gives shy fish somewhere to retreat, which pays off in calmer, healthier behaviour.
Water parameters
Target 24–28 °C (75–82 °F) and a pH near 7.5–8.5. The single most important rule is stability: a fully cycled tank with ammonia and nitrite at zero matters more than hitting an exact number on the dot. Test weekly and act on trends, not single readings. Carry out regular partial water changes to keep nitrates under control, and match the temperature of any new water to the tank so you never shock the fish. A simple log of dates and readings takes two minutes and turns vague worry into clear pattern recognition.
Feeding Molly
They are omnivore by nature, so offer a varied diet built on a quality dry staple with frozen or live foods a few times a week. Feed only what they clear in about a minute, once or twice daily for adults. Overfeeding is the fastest route to poor water quality and ammonia spikes, so when in doubt, feed a little less rather than a little more. Remove any uneaten food after a few minutes with a fine net or turkey baster. A varied diet keeps colours vivid and the immune system strong; a monotonous one leads to deficiency and faded fish over time.
Tank mates
Good companions for Molly include platies, guppies, snails, corydoras. Avoid housing them with soft-water tetras, aggressive cichlids. Always match tank mates by temperament and adult size, not just by how they look in the shop — a fish that fits the tank at two centimetres may outgrow every neighbour within a year. Add new tank mates to a settled, established tank rather than all at once, and observe for the first hour to catch any chasing early.
Common problems to watch for
Even well-kept Molly can run into trouble. The most frequent issues are parasitic and bacterial infections, poor water quality, and stress from overcrowding or bullying. Learn the early warning signs now: clamped fins, lost appetite, rapid breathing, flashing against decor, or a change in colour. Catching a problem in the first day or two is the difference between a quick recovery and a tank-wide outbreak. Quarantine new arrivals in a separate quarantine tank for two to four weeks before they join the main display.
Acclimating new Molly
When new fish arrive, float the sealed bag for 15–20 minutes to equalise temperature, then add a small cup of tank water every five minutes for half an hour so they adjust to your parameters gradually. Net them across rather than tipping shop water into your tank. Dim the lights and keep the room calm for the first evening; a gentle introduction prevents the shock that often precedes illness. Only feed the next day, once they have settled and are swimming normally.
A simple weekly routine
Keep Molly thriving with a repeatable habit: test water weekly, top up with matched-temperature water during a partial change, wipe algae from the glass, and glance at the filter flow. Watch feeding behaviour every day — a fish that skips a meal is telling you something. A calm, consistent routine prevents most problems before they start and makes the few that appear easy to spot and treat.
