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Fertilized vs. Unfertilized Neon Tetra Eggs: A Complete Guide

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The first question fish owners often ask when they discover a new batch of neon tetra eggs is whether they are fertilized.

I remember having the same question when I first encountered them a few years ago.

Over time, I’ve learned to distinguish between fertilized and unfertilized eggs and noticed common traits among fish eggs in various aquarium species.

In this article, I will guide you on how to spot these differences and provide detailed images to assist you in identifying them yourself. Let’s dive right in.

How To Distinguish Fertilized Neon Tetra Eggs From Unfertilized Ones?

CharacteristicsFertilized Neon Tetra EggsUnfertilized Neon Tetra Eggs
ColorYellowish or brownishWhite or translucent
SpottingSmall black spotsNone
ResultHatch into fry within 24 hoursDo not hatch
Fungus InfectionNoFuzzy, cotton-like white
Color RetentionOpaque over timeRemain translucent or white
ReasonPresence of males in the tankLack of males in the tank

As described in the table, there are many differences between fertilized and unfertilized neon tetras eggs. This is what you should know:

  • Egg Opacity: Fertilized eggs are clear or translucent, while unfertilized eggs become cloudy or opaque. Unfertilized eggs often develop a white or gray hue over time.
  • Shape and Consistency: Fertilized eggs usually maintain their shape and are somewhat firm to touch. Unfertilized eggs, however, may appear deflated and feel softer or mushy.
  • Egg Clustering: Neon Tetra eggs tend to be laid in clusters. Clusters with mixed coloration (clear and cloudy eggs) are likely a mix of fertilized and unfertilized eggs.
  • Presence of Embryos: As development progresses (around the 24-48 hours mark), you can observe tiny embryos inside the clear fertilized eggs. Unfertilized eggs don’t exhibit this.
  • Mold Growth: Unfertilized eggs often succumb to fungal growth much quicker than fertilized eggs. This appears as a fluffy white growth on the egg.
  • Hatch Time: Fertilized eggs usually hatch within about 24 to 36 hours at around 77°F (25°C). If eggs haven’t hatched after this time, they’re likely unfertilized.
  • Parental Behavior: Observing your neon tetras can provide clues. If both parents are tending to the eggs, this could indicate a successful fertilization.
  • Egg Disappearance: Unfertilized eggs are often eaten by the neon tetras themselves or other tank inhabitants. A reducing number of eggs could indicate this.
  • Tank Conditions: Healthy fertilized eggs are more likely in optimal tank conditions: clean water, appropriate temperature, pH levels and low light. Unfertilized eggs could indicate poor conditions.
  • Previous Spawning Success: If your neon tetras have successfully spawned before in similar conditions, it’s likely they’ll produce fertilized eggs again.

Also Read: Neon Tetra Eggs 101

What To Do With Fertile Neon Tetra Eggs?

If you spot the signs associated with fertilized eggs, simply follow these steps:

  • Separate the Eggs: Once you’ve identified fertilized eggs, gently move them to a separate, clean aquarium to prevent adult fish from eating them.
  • Ensure Ideal Conditions: Keep the temperature at 77°F (25°C) and the pH between 6.5 and 7.
  • Dim Lighting: Neon Tetra eggs are sensitive to light. Keep the tank in a dimly lit area to prevent damage to the eggs and to stimulate hatching.
  • Avoid Disturbance: Reduce movement around the tank. Vibrations can damage the eggs or stress the hatching fry.
  • Prepare for Hatchlings: Have a plan for feeding the fry once they hatch. Infusoria or commercially prepared fry foods are good options.
  • Monitor Development: Regularly check on the eggs’ development. If you see any that have become cloudy or have fungal growth, remove them immediately to prevent it spreading.
  • Regular Water Changes: Carry out regular water changes (about 20%) to maintain water quality but be careful not to create water movement that could damage the eggs.
  • Gradually Introduce Light: Once the fry have hatched, slowly introduce them to more light. This helps encourage feeding and growth.
  • Rearing the Fry: Once the fry start swimming around, they are ready for more substantial food like brine shrimp. Gradually increase the light and monitor their growth.
  • Prepare for Integration: When the fry are large enough, you can slowly introduce them to the main tank. Ensure other fish are well-fed to minimize the risk of them eating the fry.

Also Read: Are Neon Tetras Livebearers Or Egg Layers?

What To Do With Unfertilized Eggs?

Follow these steps if you suspect that your neon tetras’ eggs are not fertilized:

  • Remove Immediately: Once you identify an unfertilized egg, it’s important to remove it from the aquarium right away to prevent decay and water contamination.
  • Monitoring for Mold: Keep a close eye on your tank for signs of mold. Unfertilized eggs tend to develop mold quickly, which can spread and infect fertilized eggs.
  • Use a Siphon: A small siphon or turkey baster can be used to delicately remove unfertilized eggs without disturbing the rest of the aquarium.
  • Prevent Recurrence: Assess the conditions of your aquarium. Unfertilized eggs can indicate suboptimal conditions or health issues in your fish, so ensure water temperature, pH, and other factors are in appropriate ranges.
  • Consult a Vet: If your neon tetras frequently produce unfertilized eggs, consider seeking advice from a vet or aquatics specialist to identify any potential health issues.
  • Egg Disposal: Dispose of the eggs responsibly. Don’t flush them down the toilet or drain. Instead, throw them in your compost or regular waste bin.
  • Consider Isolation Tank: If unfertilized eggs become a frequent issue, consider setting up a separate spawning tank to easily control and clean the environment.

Why Are My Neon Tetra Eggs Not Hatching?

If your neon tetras’ eggs failed to hatch, these are usually the reasons:

  • Unfertilized Eggs: Eggs not hatching is often due to being unfertilized, caused by poor health, diet, or all-female group.
  • Tank Conditions: Improper temperature (around 77°F or 25°C), pH levels, or water quality can hinder egg hatching.
  • Excessive Light: Exposing neon tetra eggs to excessive light damages embryos, preventing hatching.

Also Read: Can Neon Tetras Lay Eggs Without A Male?

What To Do After Fertilized Eggs Hatch?

Once the eggs hatch and the fry appear, it’s necessary to take the proper measures:

  • Provide Adequate Food: Newly hatched neon tetra fry are extremely small and need tiny food like infusoria or commercially prepared fry food. As they grow, they can be fed baby brine shrimp.
  • Maintain Water Quality: Keep the water clean and well oxygenated. Perform regular small water changes, but be careful not to disturb the fry.
  • Keep Dim Lighting: Continue keeping the tank in a dimly lit area until the fry grow larger and stronger. Gradually increase light as they age.
  • Monitor Fry Development: Regularly check on the fry’s development. If any appear unhealthy or dead, remove them immediately to prevent contamination.
  • Avoid Disturbance: As with the eggs, reduce movement around the tank to avoid causing stress to the fragile fry.
  • Gradually Adjust Conditions: Over time, gradually adjust the water parameters and lighting to match the main tank conditions in preparation for integration.
  • Slow Integration: When the fry are large enough (usually about a month old), slowly introduce them to the main tank. Make sure other fish are well-fed to reduce the chance of predation.


If you are in a rush, here is a quick summary of what I discussed earlier:

  • Distinguishing between fertilized and unfertilized neon tetra eggs is crucial to understanding their development. Characteristics such as color, spotting, opacity, and the presence of embryos can help determine their fertility.
  • Maintaining optimal tank conditions, including temperature, pH levels, water quality, and lighting, is essential for successful egg hatching and fry development.
  • Unfertilized eggs are prone to fungal infections, which can be identified by fuzzy, cotton-like growth. Removing unfertilized eggs promptly is necessary to prevent contamination and mold spread.
  • Proper care and monitoring of fertilized eggs involve separate housing, ideal conditions, dim lighting, minimal disturbance, regular checks, and the removal of any damaged or infected eggs.
  • Unfertilized eggs should be removed immediately to prevent decay and water contamination. Consistent water quality and consulting a vet in case of recurrent unfertilized eggs can help address any potential health issues in the fish.