Molly caresheet

  • #1
Here is a caresheet for would-be molly owners.

For existing molly owners, please add your remarks/tips/thoughts as a new post and I'll integrate it to the main text with your name.

First off, a personal note: they are my ABSOLUTE FAVORITE fish. They are trusting, inquisitive, smart, lively, and come in a variety of colour patterns.



There are 3 different species of mollies, which have been re-classified, ungrouped and re-grouped several times. Today's consensus is as follows:

  • Peocilia velifera (also called P. mexicana) = Yucatan molly. Sometimes erroneously referred to as sailfin molly
    • size: Larger than P. latipina (about 8cm/6in for big specimens).
    • dorsal fin: Males' dorsal fin is high. It has more rays (about 18) than the latipinna. Also, upper edge of dorsal longer than lower. Dorsal fin markings are slight, small spots.
    • salt: really need brackish water to thrive.
    • other requirements: They need direct sunlight, and more gallons per fish than the latipinna.
  • P. latipinna ('true' sailfin molly).
    • size: Smaller than velifera (about 6cm/3in for big ones) and more slender
    • salt: Tolerate some salinity but do not require it.
    • dorsal fin: The males' dorsal fin is high. has less rays (about 15). Also, upper edge shorter than lower edge. Their dorsal fin is edged in a yellow line and they have dark and square markings on the dorsal fin.
  • P. sphenops (short-fin molly, e.g. black molly).
    • size: Smaller yet than the latipinna (about 6cm/3in for big males; 5in for big females) and more slender
    • salt: Tolerate some salinity but do not necessarily require it.
    • dorsal fin: Their dorsal fin is a lot less high than either sailfin type.
Some morphological varieties have been bred into the three species: you can now find balloon (pot-belly) and lyretail varieties, for example, and a whole sort of colour assortments.

Small note on balloon mollies

The balloon trait is obtained by selectively breeding fish with scoliosis (bent spine). Balloon mollies often have a reduced lifespan due to three factors: the inefficiency of the misshapen morphology causing wear and tear while swimming, the compact position of its internal organs in comparison to a non-balloon variety, and oft-seen difficulties giving birth.

I have one, which I bought unwittingly. She's now my favorite fish, but I am refraining from purchasing another balloon molly from the shops for the reasons stated above.

As mollies are little devils when it comes to breeding, this section's huuuuge.

How can I tell if it's a male or a female?

The male has a pointy-shaped analfin (gonopodium), whereas the female has a 'normal' triangular set of analfins. For sailfin mollies, the males also have a high dorsal fin.

How do I know they're mating?

The male will constantly chase the female and point his gonopodium towards her belly in the chase.

How long is the pregnancy?

Normally speaking, the gestation period is abour 4 weeks after mating. They can get pregnant immediately after giving birth, so it's not uncommon to have your females nearly PERMANENTLY pregnant, and giving birth every 4 weeks or so.

Female mollies can hold sperm without letting it develop into fry for a very long time (weeks and weeks on end). They'll do that if they feel the conditions are not right for their young to develop. This is causing the oft-seen phenomenon of females arriving pregnant form the shop, prompting many people to bewonder the miracle of immaculate conception, or extreme cross-species breeding with, say, blue whale tank mates.

What's a livebearer?

Mollies are livebearers, which means that the fry is born free-swimming. They are ovoviviparous, though, i.e. the young does have an egg stage inside the mother, unlike mammals, which are viviparous (no egg stage at all: the baby IS the egg).

How many fry will she get?

A young female molly's first ever batch can be as little as a dozen fry. As she gets pregnant again (and again, and again and again...), she's likely to give birth to increasingly large batches. Some people report prolific mothers regularly give birth to 80 or so fry per batch.

They may give birth in 2 or more phases, releasing the second half a week or so after the first batch was dropped.

How do I know she's pregnant?

A molly female about to give birth will look almost boxy in shape, distended that she is with all the fry inside her. Another tell-tale sign that she's about to give birth is that she may lack appetite a day or two before dropping her fry. She'll also likely isolate herself form the others, and lay in a quiet spot on the substrate. This behaviour often prompts panicked owners to diagnose a disease, but it's common in pregnant livebearers, and she'll soon recover her spirirts. If your fish is relatively transparent in colour, you may distinguish what is called the gravid spot on her abdomen. This looks like a darker spot, and is the mass of fry that you can see through the fish. The closer she is to dropping, the closer the spot will be to her cloaca.

Why does my fry disappear?

Mollies (and other tankmates) can eat their young. The young also commonly get trapped under gravel. If you want to keep your fry alive, you'll need to either provide plenty of cover for them (a floating plant, minI pots/jars on the substrate, stacked flat marbles or a densely planted area should ensure that some of your fry makes it to adulthood) or to isolate the female prior to her giving birth. I personally never isolate my pregnant females, as it stresses them out.

Simply speaking, the fry cease to be in danger when they are too big to fit in the tank mates' mouth.

When do I know if the fry are male or female?

You can sex the young from about 3 months old. Before that, the immature fry will all look like females.

Do separate them from their parents before they reach maturity to avoid inbreeding.

Interbreeding between species

All three molly species can interbreed. Other livebearers can also breed with mollies. The following interbreeding combinations have been reported:
But beware, the resulting fry are mostly sterile, deformed runts, and you will not be able to give them away to a shop.


Are they good community fish?

From the perspective of their temperament, I find them great tank mates. I've not really witnessed any concerning cross-species aggression to other fish. But, mollies can be prone to disease, so I like the comfort of a species only tank so that I can optimize my water chemistry to their greatest comfort.

How many per tank?

Mollies are quite the waste producers, so I'd stock under one inch of molly fish per gallon.

Male/female ratio

If you don't want fry all the time, you'll need to separate the males from the females.

Same-gender tanks should really have at least 3 mollies together to avoid the alpha fish victimising the subservient fish.

Mixed-gender tanks should keep at least 1 male per two, or even better, three, females. You'll soon notice that the male's insatiable appetite would end up exhausting a lone female.


Mollies and food, a love story

Mollies will do anything for food. They are finned stomachs. No matter how often you feed them, they'll beg.

They are mainly vegeterians, but can eat the odd protein-based food. Do avoid live food (risk of infection), unless you grew it yourself.

Their diet in the wild consists in:

- algae (they're reasonably good algae eaters, actually)
- plants (yep, sorry, some of your plants will suffer from molly nipping)
- small worms/insect larvae

Staple diet in the tank

- slow-sinking protein and vegetable pellets, served on the surface so they get to chase stuff
- a variety of vitaminated flake food promoting growth, colour and immune system
- fast-sinking protein and vegetable pellets so they have to browse for their food

And for some variety
- cucumber or zucchinI slice planted with protein pellets
- frozen bloodworms (thawed)
- spinach
- mashed, peeled and crushed pea bits
- algae pills (they loooove the stuff)
- tubifex dried cube

Food = entertainment and learning

Food time is entertainment time. Whenever you can, make it hard for them to get to their food. Also use food to bond with them (hand-feeding is a very special moment). If you use food, you have hours of fun teaching them little tricks that will keep them stimulated, and that'll entertain you no end.

How much to feed them

A common rule of thumb is that a fish' stomach is the same size is one of its eyes, so try to stick to one stomach's worth of food per molly per feeding.

Feed in slow batches, giving a small portion and waiting until they've eaten it before giving more.


I'd recommend fasting them 1 day a week


Some frequent symptoms

The following are signs that something's not right:
shimmy: get shimmy block or molly bright medication to catch at onset. Shimmy is sign of early development of one of several possible diseases.
torn fins: either result of a fight or finrot. I would isolate the fish and treat with disinfectant. If it is finrot, you will need to determine whether it bacterial or fungal before treating
flashing/scratching: water quality problem (nitrate/nitrite/ammonia) likeliest. External skin parasites also likely.
lethargy: commonest cause is just pregnancy
gasping for air on the surface: temperature could be too high. oxygenation could be too low. could be nitrate/nitrite/ammonia poisoning.
reddened/purple gills: sign of nitrites/nitrates/ammonia poisoning.
dull white film on skin: could be oodinium (turn the tank light off and shine a torch on the fish. If the white patch glows back, it's oodinium). Could be a fungal infection (furry-looking white patch, like mould)
slanted swimming, swimming upright or on the side: possibly a swim bladder problem, or a spine problem. Spine problems can be caused by a vitamin defficiency or by TB.
pop eye: Symptom of many possible diseases, including the dreaded fish TB. Could also be mechanical damage from fight or knocking against ornament. Pop eye can also be caused by water pollution with plastic or metal, or by ammonia/nitrite/nitrate. In rare cases, pop eye is caused by oxygen oversaturation (gas bubble disease)
dropsy: (means fish' midsection is extremely swollen, and scales stand upright in worst cases). Symptom of many possible diseases, including fish TB.
sunken belly: Symptom of many possible diseases, including fish TB. Sometimes the fish has trouble eating from a wound around the mouth area.
grain-like white spots: Ich.
constipation: Poo constantly stringing out of fish, or fish' belly really distended.
stringy excrement: internal parasites.

What to do with a sick fish

Try to react quick at the onset of a disease, but try to not do anything unless you are quite sure of your diagnostic.

It's always a good idea to have a small spare tank to isolate and treat sick fish in.

It's always a good idea to have a magnifying glass handy to inspect the fish from upclose.

It's always a good idea to have a first-aid pharmacy for your fish.

If you have a sick fish, don't despair. I've had some real miracle cases that were completely beyond hope, and miraculously mended themselves once I'd applied the right meds.

Caution with meds

If your pH is high or low (i.e. above or below 7.0), do check wheter to undermedicate or overmedicate (compounds become more, or less potent, depending on the combination of the pH and the compound itself).

In general, it's best to keep medicating sick fish for about 2 weeks after the last symptoms are gone. Do read the med's instructions to see whether they strongly discourage this.

For community tanks, do make sure that the non-molly species can tolerate the treatment (read medication instructions). This goes for salt and increased temp too. Be extra careful with medication containing copper, to which most bottom dwellers are very sensitive.

Some meds (in particular anti-bacterial meds) will play havoc with your cycle (they might kill your nitrites and ammonia-eating bacteria). Check the medication's instructions to see whether that is one of the side effect and if you can, only buy medication that doesnot affect your cycle.

What to do after meds: activated carbon

It is recommended to use activated carbon after the end of a course of medication because AC will absorb the medications from the tank, thus giving you medication-clear water again. Most AC brands allow you to keep the carbon in the filter for 1 week, then you'll have to remove it or the carbon will expulse all the medication back in the tank.

Common problems/diseases

Mollies are notorious for being disease-prone. The following are really common problems:
- nitrate/nitrite/ammonia poisoning
- mouth rot
- fin rot
- ich
- oodinium
- fungus infection

How to minimise the risk of disease

A relaxed environment with little human disturbance and fish aggression, and a clean water and substrate should reduce the risks.

To salt or not to salt?

Sailfin and black mollies can tolerate a little bit of salt (mexican mollies require it), up to about 1 table spoon of salt per gallon. It has been argued, however, that salt actually irritates their skin. I myself only use salt in cases of ich.

If you do use salt, please use aquarium salt and not kitchen salt (has natrium in it) or marine salt (has minerals in it).



Floating plant cover and marbles and little pots on substrate for fry

Plenty of plants (broadleaves and others)

At least one densely planted area for fry to hide in if you want fry

Tall plants breaking the view in the middle of the tank, to allow victimised fish to be in another section of the tank as (and invisible to) their bully.

Tunnels, driftwood, etc. won't go amiss with them

A lot of hiding places to mitigate the risk of bullying.

Entertainment ideas

Give them lots and lots and lots and lots of decorations and you'll have great fun watching them explore and play.

Move your decoration often, as they are inquisitive little fish and love to explore.

Give them differently arranged pieces of a hamster maze for them to check out.
  • #2
Bring this up since many members miss stickies.
There have been a lot of mollie questions.
Hope this helps.

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