Why do Swordtail Fish Eat Their Fry?

Are your baby swordtail fish mysteriously disappearing? The alarming truth behind this phenomenon lies in the behavior of their very own parents. Join me, as I unravel the reasons why swordtail fish become cannibals, eating their precious offspring, in the aquatic world.

swordtail fish feeding

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Reasons Swordtail Fish Eat Their Babies

There are many reasons why adult swordtail fish eat their babies including lack of food available, lack of space, stress due to poor water quality, or cannibalistic instinct. Now, let’s explore in detail these common reasons:

Lack of Proper Diet

Swordtail fish are omnivorous creatures that require a well-balanced diet to thrive. When these fish do not receive an adequate supply of nutrients from their regular diet, they may resort to eating their own babies to compensate for the missing nutrients. An improper diet leads to poor health and lower survival rates for both the adults and the fry.

Insufficient Aquarium Space

Limited swimming space can lead to higher aggression levels in swordtail fish. When confined to a small tank, adult swordtails may become territorial and display aggressive behaviors, such as nipping at their babies. This cramped environment can also trigger stress and force them to consume their young as a means of population control.

Safety and Predatory Instincts

Ironically, swordtail fish sometimes eat their babies to protect them from other predators present in the tank. In their minds, it’s better to devour them than letting a predator do the same. This bizarre behavior also prevents incompatible species from cross-breeding, thereby preserving their genetic lineage.

Stress Caused by Poor Water Quality

Maintaining proper water conditions is essential for the well-being of your swordtail fish. However, if water quality is poor, it can lead to stress, illness, or abnormal behavior in fish- including the consumption of their own offspring. To avoid this tragic situation, it’s paramount to monitor the water parameters and follow proper aquarium maintenance practices.

The Struggle for Dominance

Swordtail fish showcase a distinct social hierarchy within their community. This hierarchy is especially evident when multiple males compete for dominance within the tank. They may eat their own offspring to eliminate competition and assert their dominance over others.

In conclusion, there is no single reason why swordtail fish engage in cannibalism. A combination of factors, such as unnatural instincts, improper tank conditions, and poor diet, contribute to this undesirable behavior. 

red female swordtail fish

Prevent Swordtail Adults from Eating Their Fry

If you want to avoid adult swordtail fish from eating their babies, there are a few things you can do about it. Here are some tips on saving swordtail fish fry:

Feed Adult Swordtails Regularly

Feeding adult swordtails regularly and with the proper diet is essential in keeping them healthy and reducing the chances of cannibalistic behavior. A well-fed adult is less likely to see its offspring as a potential food source. Here are some tips to feed adult swordtails effectively:

  • Offer a Balanced Diet: Provide them with a variety of foods, such as high-quality flakes or pellets, live and frozen foods (like bloodworms, brine shrimp, or daphnia), and blanched vegetables (zucchini, cucumber, and spinach). Mixing various food sources keeps their diet interesting and ensures they receive all the necessary nutrients.
  • Feed in Small Portions: Swordtails benefit from smaller, frequent meals. Offer food portions that can be consumed within 2-3 minutes, 2-3 times a day. Overfeeding can lead to obesity, which affects their health and life span. Plus, leftover food may deteriorate water quality, leading to stress and other health issues.
  • Diversify the Menu: Change the food source from one meal to another. Maintaining diverse eating habits keeps swordtails more interested in their diet and keeps them from becoming picky eaters. Variety is key to keeping your fish engaged, healthy, and nutritious.
  • Monitor Their Diet: Pay attention to the eating habits of your swordtails to make sure they’re receiving the right amount of food. If you notice a fish that’s consistently rejecting food or appears unhealthy or malnourished, it may signal an underlying health issue that needs to be addressed.
  • Set a Routine: Swordtails appreciate a consistent feeding schedule. Establish and maintain specific feeding times, allowing your fish to adjust and anticipate their next mealtime to prevent cannibalism.

By implementing these practices and ensuring the regular feeding of adult swordtails, you can greatly reduce the likelihood of them eating their young. An appropriate diet and proper feeding routine contribute significantly to your fish’s overall happiness and longevity.

Use a Breeding Box

breeding box provides a safe and controlled environment that significantly increases the chances of swordtail fry survival. These boxes separate pregnant females from other tank inhabitants, preventing possible consumption of the fry upon birth. Here’s a guide to effectively using breeding boxes:

  • Choose the Right Breeding Box: Breeding boxes come in various shapes and sizes. Some are designed as nets or mesh, while others are made of plastic. Opt for a box large enough to house a pregnant swordtail comfortably, ensuring she has the necessary space to swim and give birth with minimal stress.
  • Consider Water Circulation: Some breeding boxes allow water to circulate from the main tank, maintaining a consistent water temperature and filtration. This feature is essential in keeping the box clean and providing a stress-free environment for the expectant female.
  • Introduce the Pregnant Swordtail at the Right Time: Timing is crucial when placing a pregnant female in the breeding box. It’s best to introduce her when she’s about to give birth, usually evidenced by a noticeably swollen belly behind the dark gravid spot. Keep an eye on these signs to prevent placing her in the box too early.
  • Monitor the Female: Check on the expectant swordtail regularly to ensure she’s handling her temporary confinement well. Some fish may become too stressed in the breeding box, leading to complications or cannibalism. In such cases, consider alternative methods, such as a separate breeding tank or ample hiding spaces for the fry.
  • Remove the Female After Birth: Once the swordtail gives birth, gently remove her from the breeding box and return her to the main tank. Leaving her with the fry for an extended period may still pose a threat to the baby swordtails.

Using a breeding box is a practical, effective method to prevent swordtail adults from consuming their fry upon birth. Providing a secure, nurturing space for pregnant swordtails ensures the survival and growth of their offspring without exposing them to potential harm.

Separate Pregnant Female

Isolating pregnant swordtail fish in a separate tank is a popular and effective solution to prevent adults from consuming their offspring. This approach provides a stress-free environment for the expectant mother and ensures a higher survival rate for the fry. Here’s how to properly separate and care for a pregnant swordtail:

  • Set Up a Separate Aquarium: Prepare a separate, smaller tank specifically for breeding purposes. The new tank should carry the same water conditions (temperature, pH, and hardness) as the main tank to minimize stress upon relocation. Additionally, introduce some plants and hiding spaces for the fry to take shelter after birth.
  • Identify the Pregnant Swordtail: Keep an eye on your swordtails to notice physical changes indicating pregnancy, such as a swollen belly and a prominent gravid spot. The gravid spot is a dark, triangular marking visible behind the female’s belly when it carries fertilized eggs.
  • Carefully Transfer the Expectant Swordtail: Gently net the pregnant swordtail from the main tank and release her into the new breeding tank. Be cautious not to injure the fish or expose her to abrupt water condition changes that might cause unnecessary stress.
  • Monitor the Female During Pregnancy: Regularly observe the expectant swordtail to ensure she remains healthy and stress-free throughout her pregnancy. Maintain proper water conditions, provide enough hiding spots, and offer a well-balanced diet.
  • Post-Birth Care: After the female gives birth, transfer her back to the main tank. Leaving her with the fry longer than necessary may encourage cannibalism or negatively impact her well-being.

Separating a pregnant swordtail in a safe and comfortable breeding tank ensures the well-being of both the mother and her offspring. This approach significantly increases the fry survival rate by providing a secure environment for their growth and protection from potential predators, including their own parents. 

Use Live Aquatic Plants for Hiding

A natural and aesthetically pleasing method to protect swordtail fry from cannibalistic adults is by using live aquatic plants. These plants provide crucial hiding spaces for the fry, helping them avoid predation while enriching the overall aquatic environment.

Here are some tips for using live plants in your swordtail tank:

  • Select Appropriate Plant Species: Choose plant species that thrive in your swordtails’ tank conditions. Anubias, Amazon Sword, Java Fern, and Java Moss are popular choices as they provide adequate coverage and adapt well to a wide range of water parameters.
  • Diversify the Plant Types: Incorporate a mix of rooted and floating plants, which serve different purposes within the tank. Floating plants like Duckweed and Hornwort provide shelter near the surface, while rooted plants like Cryptocoryne and Vallisneria offer coverage at different depths.
  • Strategically Position the Plants: Arrange the plants carefully within the tank, ensuring that there are ample hiding spots throughout the aquarium. Intersperse taller plants with shorter ones, creating a diverse and protective environment for the fry.
  • Maintain the Plants: Regularly prune and trim the plants to keep them from overgrowing or overcrowding the tank. Proper maintenance ensures that the plants remain healthy and continue to offer the needed protection to the baby swordtails.
  • Consider Artificial Plants: While live plants offer additional benefits like oxygenation, nutrient absorption, and aesthetics, artificial plants can also be a suitable hiding spot option. If you’re unable to maintain live plants, opt for artificial plants that mimic natural ones without the maintenance requirements.

Incorporating live aquatic plants within your swordtail tank not only creates a visually appealing environment but also provides essential hiding spaces for the delicate fry.

By diligently selecting, positioning, and maintaining live plants, you can protect your swordtail fry while enhancing the overall quality and appearance of your aquarium.

koi female swordtail fish

Keeping Different Sizes of Swordtails Together

While keeping swordtail fry safe from their parents is a priority, it’s also important to address the issue of intra-species aggression among swordtails of varying sizes. A harmonious environment can help minimize the chances of cannibalism within the community.

Follow these guidelines for successfully maintaining a peaceful tank with swordtails of different sizes:

  • Plan the Tank Layout Wisely: Design your tank keeping in mind the size differences among your swordtails. Ample swimming space and plenty of hiding spots (using rocks, driftwood, and plants) will help the fish coexist without feeling threatened by each other.
  • Monitor Tank Dynamics: Be attentive to the interactions among your swordtails, taking note of any aggressive behavior or territorial disputes. Identifying and addressing such issues early will help you maintain harmony within the tank and reduce the likelihood of cannibalism.
  • Gradually Introduce New Swordtails: When adding new swordtails of different sizes, acclimate them gradually to prevent stress and aggression within the tank. Start by floatation acclimation, followed by adjusting the fish to the tank’s water parameters, and finally releasing them after turning off the tank lights to minimize confrontations with the established community.
  • Maintain a Proper Female-to-Male Ratio: A balanced sex ratio is crucial in managing aggression between swordtails. Aim to maintain at least two to three females for every male swordtail. This ratio helps reduce competition among males for mating and prevents potential conflicts in the population.
  • Consider Tank Mates: Introducing compatible non-swordtail species can help create a sense of balance within your aquarium community. Peaceful tank mates, such as platies, mollies, rasboras, or tetras, can coexist with swordtails without contributing to cannibalism, further diversifying your tank.

By being proactive in keeping swordtails of different sizes together and addressing the factors that contribute to aggression and cannibalism within the tank, you can create a harmonious and thriving aquatic ecosystem.

Proper design, right ratios, and diverse, but compatible tank mates will help ensure the safety of your swordtail fry and the overall health of your aquarium community. 


Now that we’ve explored the reasons behind swordtails eating their babies and effective methods to prevent it, you can provide a safe and nurturing environment for your fish. Have you experienced this issue? Let us know your thoughts and solutions in the comments!

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