The Book of Household Management by Mrs. Beeton

221. FISH SHOULD BE PUT INTO COLD WATER, and set on the fire to do very

gently, or the outside will break before the inner part is done. Unless the fishes are small, they should never be put into warm water; nor should water, either hot or cold, be poured _on_ to the fish, as it is liable to break the skin: if it should be necessary to add a little water whilst the fish is cooking, it ought to be poured in gently at the side of the vessel. The fish-plate may be drawn up, to see if the fish be ready, which may be known by its easily separating from the bone. It should then be immediately taken out of the water, or it will become woolly. The fish-plate should be set crossways over the kettle, to keep hot for serving, and a clean cloth over the fish, to prevent its losing its colour.