These fish are near-albino or ‘xanthic’ – an absence of melanin in the skin allows the blood vessels to show through, but the eyes remain pigmented. Any form of albinism usually results in a degree of weakness in the animal, and inbreeding near-albino fish to produce a ‘red strain’ gives rise to ‘inbreeding depression’. This genetic condition is caused by breeding from a limited gene pool, and results in a reduction in general health.