Which organ is mainly responsible for the removal of ammonia from the body in fishes?

Gills are responsible for the removal of ammonia from the body in fishes.

Ammonia is a waste product of protein metabolism, from the body, which needs to be eliminated to maintain the homeostasis in fish. As water passes over the gill filaments, it comes into close contact with the blood vessels within the filaments. Along with oxygen uptake, gills also facilitate the elimination of waste products, primarily in the form of ammonia. The process of ammonia removal in fish gills involves diffusion. Ammonia diffuses from the blood in the fish's body into the water flowing over the gills. This allows the fish to excrete excess ammonia out of its body, effectively eliminating this toxic waste product.