Development
If the eggs are fertile, and kept in darkened surroundings, they will hatch in about three days at 28°C. Free-swimming fry remain photosensitive for at least the first seven days of life, and need to be introduced to increasing light levels on a gradual basis. During this time, they are approximately 4 mm in length, and require infusoria or liquid fry food. Newly hatched brine shrimp and other similar live foods, such as sifted Daphnia, can be fed to the growing fry at between seven and 14 days of age. Growth continues at a modest rate, and the fishes assume full adult colouration only after a period of around eight to 12 weeks, depending upon quality of food and aquarium water.
The characteristic iridescence of this and related fishes, such as the neon tetra, is a structural color, caused by refraction of light within guanine crystals that develop within special cells called iridocytes in the subcutaneous layer. The exact shade of blue seen will depend on the viewing angle of the aquarist relative to the fish - if the aquarist changes viewpoint so as to look at the fish more from below, the colour will change hue, becoming more deeply sapphire blue and even indigo. Change the viewpoint to one above the fishes, however, and the color becomes more greenish.
Read more about this topic: Cardinal Tetra
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—Mark Twain [Samuel Langhorne Clemens] (1835–1910)