There are granular white spots at the bottom of the fish tank, the size of which is like small grains of sand.

Probably soda water. I don't know how often you clean the bathtub, my friend. Fish tanks should be cleaned regularly. Koi fish has great vitality. You should feed it with old water instead of twice a day. This kind of fish can eat and pull. If you don't want to keep changing water, don't keep feeding it.

I hope it helps you.