Hatching Problems
A small percentage of eggs will fail to develop, usually because they were not fertilised in the first place. Dead (opaque) eggs rapidly develop a fluffy appearance due to fungal infection and should be removed regularly, without disturbing the rest of the spawn, to avoid the spread of infection.
Chemical treatment can be used to limit the infection. Many different chemicals are used by breeders with varying degrees of success. These include acriflavine, methylene blue, and other dyes. I always had success with methylene blue. Add sufficient methylene blue to produce a concentration of 3 ppm. One treatment is usually all that is necessary, and should continue for at least 3 days. Chemical treatment with methylene blue can be administered at any time after the eggs are waterhardened, until one or two days before hatching is anticipated. Eyed-eggs are generally not affected by fungal infections.
Very high fungal infections within the first 2 or 3 days after spawning typically indicate a high percentage of infertile eggs. This may be due to unfavourable conditions within the spawning tank or adult infertility. A sudden upsurge in losses later usually signals improper water conditions in the hatching container, or egg collection damage. Eggs can also be very sensitive to changes in pH, dissolved oxygen, and temperature.
It is suspected that vertical transmission (transmission from parent to offspring) of disease can occur through rainbowfishes eggs. Vertical transmission of pathogens allows the continuous and simultaneous spread of a disease from one generation to another. Although they are not affecting the adult fish, these organisms can have devastating effects if transferred to the larvae. The transfer from one generation to the other can happen in two ways: on the egg and in the egg. For most pathogens however, the route of infection is on the eggs and then to the larvae when they hatch. Disinfection of the eggs with iodine can be carried out for the various fish species but it is most commonly used in aquaculture. For rainbowfish species, preliminary tests should be conducted to determine when and at what concentration disinfection can be carried out safely.
A problem often encountered when raising rainbowfish larvae is a little freshwater cnidarian called hydra. They are usually tan or brown in colour and are not readily seen against a background of natural coloured gravel or on plants. Within the confines of a small nursery aquarium, these little monsters can be deadly, and can ingest a batch of newly hatched rainbowfish larvae in less than a week.
Another pest that you may encounter is a free-living flatworm known as Planaria. They are a very small black or brown flatworm that look very similar to leeches and often appear in freshwater aquariums. They are generally around 3 to 5 mm long, but some grow as large as 10 mm. In a normal aquarium situation they usually don't cause any problems and probably even go unnoticed. However, if you get them in a breeding or hatching aquarium, they can destroy a whole spawning of eggs within hours. They can usually be seen crawling around on the front of the aquarium at night time after the lights have been turned off.