Methylene blue and incubation: protecting fish eggs

Why do eggs go mouldy?

In the wild, the parents often clean the eggs, or the water current is enough to oxygenate them and carry off the spores. In an aquarium, the density of fungal spores is higher. When an egg dies (unfertilised or damaged), its protective membrane breaks down, providing an ideal nutrient bed for aquatic moulds. These fungi develop a characteristic white fuzz that spreads rapidly.

Methylene blue acts as a powerful antifungal that inhibits the germination of these spores without (generally) crossing the membrane (chorion) of a healthy, living egg.

Artificial incubation protocol

This is the safest method for maximising the hatching rate. It consists of removing the eggs from the community tank and placing them in a dedicated container.

Equipment

• Small tank or incubation container (1 to 5 litres).
• Air diffuser (air stone) for vigorous oxygenation.
• Heater (if required for the species).
• Methylene blue solution (see preparing solutions).

The "deep blue" dosage

For eggs, a higher concentration than that tolerated by free-swimming adult fish is often used, because the egg membrane provides temporary protection.

1. Fill the tank with water from the original tank (same physico-chemical parameters).
2. Add the blue: aim for a very deep blue colour, in which the bottom of the container is barely visible.
   • Indicative dosage: around 2 to 3 mg/L (i.e. roughly 3 to 5 drops of a 2-5% solution per litre of water). The water should be distinctly blue, not merely tinted.
3. Introduce the eggs (on their spawning support, leaf, or mop).
4. Aerate: place the air stone right next to the eggs to create a constant current.

Monitoring and hatching

• White eggs: despite the blue, unfertilised eggs will remain an opaque white or will take up the blue (a sign that they are dead and permeable). Healthy eggs stay translucent or amber.
• As soon as hatching begins: this is the critical moment. The larvae (fry with a yolk sac) are far more sensitive to chemicals than the eggs.
  • Immediate action: as soon as the tails start to wriggle, carry out large water changes (50% to 80%) with fresh water at the same temperature to dilute the blue. The aim is to remove the product before the fry begin to swim freely and to breathe actively through their gills.

Incubation in the community tank: is it feasible?

Treating an entire tank (100 L, 200 L) just to protect one clutch is not advisable.

1. The biological filter: methylene blue attacks the beneficial bacteria in the filter (see filtration).
2. The plants: they are liable to suffer from the reduced light and from the chemical action.
3. Cost and cleaning: colouring 200 litres of water for a few eggs is inefficient and will stain the silicone seals.

If you cannot move the eggs, isolate them in a "breeding net" or a plastic breeding box inside the tank, but the effectiveness of local treatment is virtually nil because the blue will instantly dilute into the main volume.

Risks and contraindications

• Sensitive species: some scaleless or highly sensitive fish (such as certain wild Corydoras or Loricariids) may be sensitive to the blue even at the egg stage. In that case, halve the dose or use Catappa leaves (natural tannins) as a gentle alternative.
• Post-hatching overdose: if the fry swim in too concentrated a solution for too long, they may suffer respiratory distress or gill damage. Dilution after hatching is imperative.

To find out more about the mistakes to avoid, see our page on common mistakes in aquarium keeping.