The Book of Household Management by Mrs. Beeton

219. IN DRESSING FISH, of any kind, the first point to be attended to,

is to see that it be perfectly clean. It is a common error to wash it too much; as by doing so the flavour is diminished. If the fish is to be boiled, a little salt and vinegar should be put into the water, to give it firmness, after it is cleaned. Cod-fish, whiting, and haddock, are far better if a little salted, and kept a day; and if the weather be not very hot, they will be good for two days.