Without acid as a counterbalance, the alkali that is probably in the pot's content would chemically react with the flavone pigment in the flesh, giving the pigment, and therefore the flesh, an unattractive, yellowish, off-white hue. If just enough acid is added, the pH factor of the contents of the pot will be 7.0 (that number denotes the neutralization of the opposing effects of the acid and alkali). If yet more acid is mixed in with the poaching liquid, the pH factor will fall below 7.0 (signifying acidic ascendancy). When that happens, the flavone pigment becomes even whiter than it was when you procured the fish.
** Asian Recipes