Fish actually sleep.
Not in the same manner that we understand, but they do sleep. Fish do not have eyelids so they are unable to close their eyes. Instead, fish catch periods of rest by floating in one place or nestling into a cozy spot at the bottom of your pond.
Koi show stress by blushing red in their fins and on their bodies. This is caused by a stressful environment, such as poor water quality. It’s their way of showing you, their caretaker, that something is wrong.
They have teeth, my dear.
Koi are equipped with rather large teeth at the back of their throat. They do not use them defensively or aggressively but rather to process any hard-to-chew food they come across at the pond bottom.
Boy or girl?
Female koi tend to have rounder bodies and smaller, rounded pectoral fins while male koi are larger, have a sleeker shape, and their pectoral fins are larger and pointed.
Koi hear through a type of amplifying system called a Weberian apparatus that other fish do not have. It consists of four pairs of bones called ossicles that connect the inner ear to the swim bladder. The connection of the air chamber to the inner ear greatly improves their ability to hear.