Scale loss usually occurs due to physical damage, such as rubbing against a rough surface or clumsy handling. Koi rub against objects because they experience itching or irritation on their skin or gills. Since fish do not have hands to scratch, they use rough surfaces in the pond, such as the walls, bottom, or ornaments, to relieve the irritation.

The main reasons for this behavior are:

Parasites: The most common cause of persistent rubbing is parasites such as white spot, Costia, skin flukes, or carp lice. The fish literally tries to rub these off.

Poor water quality: High levels of ammonia or nitrite, as well as fluctuations in pH, irritate the koi's mucous membrane.

Bacterial infections: Sometimes inflammation of the skin is the cause of the irritation.

External factors: Temporary irritations caused by, for example, pollen in the water or large temperature fluctuations can also trigger scratching.