Why Aren’t My Mollies Breeding? (Finally Answered)

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Whether it’s Black Mollies, Gold Mollies, or Sailfin Mollies, Molly fish have long been popular in the fish keeping hobby. They are hardy, super active, and usually easy to breed. There are however occasions when Mollies won’t breed, or appear not to be breeding.

Assuming you have at least one male and one female Molly, it is very unlikely your mollies aren’t breeding on a regular basis. It is far more likely they are breeding, but the babies are being eaten by the adult mollies themselves, by other fish in the tank, or being sucked into a filter.

I have been keeping and breeding Mollies for over 20 years. I currently have 6 different species of Molly in my fish room. They are one of my favorite live-bearing fish.

How Do You Sex Molly Fish?

Mollies are live-bearing fish. The female fish retains the fertile eggs inside her body and she releases the baby fish as free-swimming fry when they are ready.

As a livebearing fish, the male has to have a way to fertilize the eggs inside the female. To do this he uses a modified analfin, which is known as a gonopodium. By comparing the analfins of our mollies, we can work out which are male and which are female.

Male mollies have a pointed analfain whereas female mollies have an analfin which is much more rounded. See image.

Why Aren’t My Mollies Breeding?

If your mollies truly are not breeding, there may be a number of different reasons. These may include;

  • Fish are not mature enough
  • Fish are in poor condition
  • Poor water quality
  • Incorrect water parameters
  • Females may be being harassed

Fish are not mature enough

Like so many live-bearing fish, mollies can breed from a very early age. Male mollies usually have to be around 12 months of age before they are mature enough to breed. Females can be as young as 6 months, but 8 to 12 months is more usual.

When I first started keeping mollies, I was always busting for them to start breeding. These days however I prefer to grow them up, feeding them lots of live and frozen foods to build as much fat as possible while they mature. I have found that, in the long run, waiting until my mollies are mature enough to breed pays dividends in the future.

Fish are in poor condition

Mollies need to be in top condition to breed, especially the females. Growing 20 to 50 babies inside her belly takes a lot out of the female. If she is in anything less than top condition, she may well abort and absorb the babies halfway through her pregnancy. This is nature’s way of making sure the female doesn’t exhaust herself and die during pregnancy.

If your mollies are in poor condition, feed them a varied diet which includes good quality flake or pellet food as well as lots of live or frozen foods. The condition of your mollies will quickly improve.

When planning to breed my mollies, I like to feed them live or frozen bloodworms at least once a day for three or four weeks before breeding.

Poor Water Quality

Although known as a hardy species of fish, mollies can still be affected by water quality. If your aquarium is suffering from an Ammonia or Nitrite spike, it may cause your female to abort and reabsorb her babies.

Even if you are running a decent filter (I like to use a Fluval hang-on-back filter on my molly tanks like the ones I wrote about in this article titled Are Fluval Filters Good?) you may still suffer from high levels of Nitrates. High Nitrate levels can affect your Mollies’ ability to regulate fluids in their bodies.

The stress this causes can reduce their desire to breed.

Incorrect Water Parameters

Much like when their water quality is poor, if their water parameters are incorrect, it can affect your mollies’ desire to breed.

Ideal water parameters for mollies are as follows;

  • Temperature: 72°F to 80°f (22°C and 26°C)
  • pH: 7.0 to 8.2
  • 20-35 KH

If your water parameters are outside of these, adjusting them back in line can start your mollies breeding again. If you don’t already have one, get yourself an API Master Test Kit to test the parameters of your water. I have found the API Master Test Kit to be the most reliable for the money. I bought this one from Amazon for less than $25.

Female Molly is constantly being harassed

Male Mollies are continually looking to breed. They will even try to breed with a female that is already pregnant.

If you only have one male to one female, or worse, more than one male per female, you may find the constant attention is causing your female to abort and reabsorb her unborn babies.

Stress is a massive cause of female Mollies reabsorbing their babies.

Are Your Molly Babies Being Eaten

Unfortunately, it turns out that Molly babies are extremely tasty. Just about every fish we keep in our community aquariums, including Mollies themselves, want to eat Molly fry.

Mollies take no parental responsibility for their babies. From the moment they are born, the fry are on their own.

Luckily they have an inbuilt instinct from birth to swim and hide. If we are breeding Mollies in a community aquarium, it is important to provide as many hiding spots as possible so the baby Mollies have somewhere to hide after birth.

It doesn’t matter what you use to provide hiding spaces – real plants, plastic plants, or a SpongeBob Pineapple house, it doesn’t matter to the baby Mollies, they just need to get out of sight. I have had really good luck using these fake plants which only cost me around a few dollars.

Are The Molly Babies Getting Sucked Into The Filter?

Most people don’t realize just how many baby fish can get sucked into the intake of a canister filter or hang-on-back filter. The strainers on the end of an intake are great for keeping adult fish out, but babies will just slip straight through.

It is possible that your Mollies are breeding regularly, but the babies are being sucked into the filter before you get a chance to see them.

To prevent this, consider placing an intake sponge over the filter intake. The sponge will not only keep the babies from getting sucked into the filter but also prevent uneaten food and plant leaves from getting drawn in.

In Conclusion

Mollies are normally prolific breeders. As a live-bearing species of fish, Mollies usually reproduce quickly and willingly in the aquarium. If you don’t think your Mollies are breeding, firstly check your Mollies are old enough to breed, and in good condition. Next, look at their environment and make any adjustments required.

Finally, if everything else seems to be right, check your baby Mollies aren’t being eaten either by other fish or by your filter.

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James

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