My fish is on the bottom of the tank and his gills, fins, and mouth is moving, oh, and, he’s starting to turn black.
Is he dead? Or is he dying?
If that happens it is because some factor it is not appropriate. It may be that the water is not cleaned or that the fish tank is too small, or that there are more fish than the account for the space of the tank. Also, it is important to clean any leftovers of blood worm from the water.
You should change the water slowly and get in touch with a store specialized in fish for the treatment of rottenness. There are several products, and not all stores have them.
All the best,