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What Do Molly Fry Need? (Filter, Heater, Lights & More)

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Most fish owners start working on their aquarium nursery as soon as they realize that their molly fish is pregnant. That immediately raises one question: what equipment do they need? Do I have to get a heater, a filter, a lighting system, etc.?

This table summarizes what equipment molly fry need:

EquipmentHow necessary?
FilterVery
HeaterVery
Air stoneSomewhat
LightsSomewhat
Air pumpNot necessary

As we move forward, I will elaborate on these different devices. And for the equipment that is crucial for molly fry, I will mention my personal recommendation.

One week-old molly fry, swimming at the top of the tank.

Still curious? Feel free to check my complete guide on molly fry. There, I discussed how to care for molly fry, what they eat, how often to feed them, their growth stages, and much more.

Do Molly Fry Need A Filter?

Yes, molly fry need a filter. As I discussed in another article, about fifty percent of the molly fry won’t survive. But if you don’t have a filter, you might lose them all. 

Every aquarium requires a chemical, mechanical, and biological filter. Some aquarists ignore the chemical filter. 

However, the biological filter is essential because it processes waste, turning ammonia into harmless compounds.[1]

The mechanical filter is just as important because it traps pollutants like leftovers and leaves, removing them before they can rot.

Some aquarists are tempted to ignore filters where molly fry are concerned because the creatures are so small. They forget the voracious appetites that make baby fish such a challenge. 

That becomes even more noticeable when you realize that a single pregnant molly fish can have up to 100 fry in a single batch

Molly fry will eat as much food as you can give them. It is common practice to feed the creatures five or more times a day

Naturally, because the babies require so much food, leftovers and waste are going to present a challenge. Without a filter, the ammonia concentration will spike, killing all the babies before they can reach adulthood.

What Is My Recommendation?

As for a filter for a molly fry nursery, I would go with the UPETTOOLS Aquarium Biochemical Sponge Filter (link to Amazon).

Molly fry can easily get sucked up by a hang-on-the-back or canister filter. But that cannot happen with a sponge filter, as they have nothing to get sucked into.

Also, this filter features a lot of surface area for bacteria to grow on. Picking the right filter is one of the main steps of keeping molly fry alive, as I previously discussed.

A picture of a molly fry eating algae from a plant.

Do Molly Fry Need Oxygen and Aeration?

Fish cannot survive without oxygen. A team from the Scripps Institution of Oceanography identified two fish species (Catshark and Cusk Eel) that survived and even thrived in waters with almost no oxygen.[2]

Unfortunately, molly fish do not have that advantage. They breathe oxygen just like most other creatures. Therefore, you cannot keep their babies in oxygen-deficient tanks.

A Conservation Physiology study from 2019 found that adult fish could tolerate low-oxygen conditions. But that tolerance was limited. After a few days, the health of the fish began to deteriorate.[3]

You cannot expect molly fry to tolerate hypoxia for days, as they are too small and sensitive. You should keep them in a well-aerated tank. Otherwise, the oxygen deficiency will kill them.

If they survive, the oxygen deficiency will either stunt their growth or saddle the creatures with various illnesses. You don’t have to pair every aquarium with an air pump.

A filter is more than sufficient. Anything that agitates the water will do because it enhances the gaseous exchange at the surface.

What Is My Recommendation?

If you already have a filter, you don’t really need anything else to agitate the water. However, if your tank is quite large or there are more than fifty fry, you probably need an air stone.

In that case, I would pick the well-known Hygger Aquarium Air Stone (link to Amazon). That is the device I use in my current tank. It is extremely quiet and gets the job done exceptionally well.

A molly fry nursery that contains about 100 fry.

Do Molly Fry Need A Heater? Can They Live Without It?

Molly fish are tropical creatures, so you cannot keep them in a cold tank. Many aquarists use the ambient temperature to maintain the conditions in the aquarium, but that approach is risky because you cannot trust the weather.

What happens in the summer when ambient temperatures spike? What about the winter? What if the ambient temperature plummets unexpectedly? You cannot afford to expose baby mollies to freezing conditions. 

Their parents may tolerate the wrong temperature for a few days, but you cannot expect molly fry to manifest the same resilience. They will die unless you add a heater to their tank.

A heater gives you more control. You can keep the temperature within the 72 – 84-degree range regardless of the weather.

Don’t forget: even if you can trust the sun to shine all year round, nighttime will eventually come. And once that happens, the aquarium’s temperature will drop, killing the fry.

If the ambient temperature is insufficient, many aquarists keep their aquariums next to the window to maximize the heat. 

But direct sunlight will only serve to increase the algae population. Don’t be surprised if algae overrun the aquarium. 

What Is My Recommendation?

If you are looking for a heater for your molly fry tank, I highly suggest getting the Orlushy Submersible Aquarium Heater (link to Amazon). 

Ideally, you don’t want the temperature to swing by more than 0.5 degrees F throughout the day. With this heater, the fluctuations didn’t go over 0.3 degrees F. Just make sure you pick the one that suits your tank’s size.

Do Molly Fry Need Light?

Mollies are not particularly complicated where lighting is concerned. On the one hand, they do not require light to get by. Don’t compare them to plants, which have a biological need for light.

Fish can technically survive without light. However, to keep them happy, you must replicate the conditions they encounter in the wild. Mollies receive plenty of light in the wild.

However, that light is not constant. The sun rises and then sets. Many aquarists endeavor to recreate that transition in aquariums. First of all, they will give the molly fish 10 to 12 hours of sunlight.

And then, once night falls, they will gradually shift to darkness. Keep in mind that the sun sets in the wild. It doesn’t switch off suddenly. Aquarium lights can produce a similar effect in the tank.

Where molly fry are concerned, you should avoid excessive lighting. That is not limited to dangerously bright lighting. 

The duration is just as important. They need several hours of darkness at night to rest. Without that darkness, the stress will kill them.

Initially, you will observe disturbing symptoms such as loss of appetite and persistent hiding. If the lighting conditions in the aquarium remain unchanged, the babies will die.

This is why professional aquarists emphasize the importance of maintaining a proper day/night cycle. The babies require both daylight and darkness to thrive. Aim for 1.5 watts per gallon, especially if your tank has plants.[4]

What Is My Recommendation?

For a lighting system, I would pick the MingDak Submersible LED Aquarium Light (link to Amazon). What I like about this device is that it has an on/off timer, so you don’t have to remember to switch the lights.

This device is also pretty affordable compared to other aquarium lights. That is a huge advantage as fish fry nurseries are usually temporary.

One week-old molly fry, searching for food on driftwood.

Do Molly Fry Need An Air Pump?

Fish live underwater. However, they still need oxygen, which is why they have gills that extract it from the water. For that reason, air pumps sound like a no-brainer.

After all, oxygen enters the water through the surface, and air pumps enhance that process by agitating the water.

However, molly fry can survive without air pumps if the aquarium has a filter, just like their adult counterparts.[5] Filters can also boost the gaseous exchange at the surface by agitating the water.

That being said, if you have the means, you should add air pumps and air stones to your nursery tank. Molly fry are vulnerable. Therefore, they are more likely to die from oxygen-deficient conditions.

Additionally, certain situations can make air pumps a necessity. For instance, if you have a large aquarium, you can’t trust the filters to maintain sufficient oxygen levels.

Medicated aquariums are equally problematic because some drugs cover the surface with a film that interferes with the gaseous exchange.

Admittedly, aquarists rarely medicate their nursey tanks. However, that doesn’t mean you can get by without an air pump. What about the plants? How many plants do you have, and how much light do they get?

Plants will only generate oxygen during the day. At night, in the dark, they will consume oxygen and produce CO2.[6]

In other words, dense foliage in a nursery tank can cause oxygen deficiencies. An air pump allows you to improve oxygen levels without reducing the density of the foliage.

A molly fish fry and a mystery snail are caught in the same picture.

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Conclusions

The two things that a molly fry nursery must have is a filter and a heater. Since these creatures eat a lot and produce a lot of waste and ammonia, a biological filter is critical.

Also, these delicate creatures are sensitive to temperature swings, so a high-quality heater is mandatory. Ideally, they thrive in temperatures ranging from 75 to 80 degrees F. 

An air stone is nice to have, especially if the tank is overcrowded. And aquarium lights are a nice touch if your aquarium is too far away from the window. Ultimately, molly fry rarely need an air pump, especially if you already have aeration.

References

  1. https://users.cs.duke.edu/~narten/faq/hardware.html
  2. https://scripps.ucsd.edu/news/research-highlight-waters-nearly-oxygen-free-researchers-find-thriving-fish-populations
  3. https://academic.oup.com/conphys/article/7/1/coz058/5644426
  4. https://animals.mom.com/lighting-mollies-6714.html
  5. https://fishkeepingguide.net/molly-fish/can-molly-fish-live-without-an-air-pump/
  6. https://www.freshwatersystems.com/blogs/blog/what-is-an-aquarium-air-pump-and-is-it-necessary