Developmental problems and hereditary disorders
Cross breeding fish between two different species is one of the main causes of hereditary disorders. A second cause of hereditary disorders is caused by constant in-breeding of closely related fish. The third source of genetic harm leading to hereditary discorders is when eggs or fry are exposed to heavy metals such as copper or lead or exposed to insecticides.
Developmental problems occur when eggs and fry are left to develop in unsuitable or unstable conditions. Fluctuating temperatures can lead to deformed spines in fry. Cold air above a tank containing labyrinth fish fry will harm the air breathing organism in the young fry.
Because malachite green contains copper it should not be used in water containing fish eggs or fish fry. Methylene blue has similar properties to malachite green, but since it does not contain copper, it is a much safer alternative.
Also, chlorine in tap water is irritating to fish and moreover, it is harmful to eggs and fry and may cause hereditary damge. So it is a good idea to age tap water for at least 7 days before being used near a breeding tank containing eggs or fry or adults that will soon spawn.
Any deformed or malformed fish should be painlessly culled as soon as possible. It is better that these types of fish do not suffer a pointless life. If they are allowed to live and grow they may breed and perpetuate their malformality. And culling these fish painlessly is better than what nature would do. Nature would cull these fish in a more brutal way.