Water makes up 90% of the body.
The most likely explanation for the variation is the offspring were produced from different combination of genes. A single pair of gold fish mentioned in the question means a male and a female gold fish. The two of them mated and contributed different genes to the fertilized fish eggs, this results in production of various body shape and colouration which is known as variation.
Somatostatin from hypothalamus inhibits the glands growth hormone
Not enough info, but I'd go with:
B) hydrogen peroxide
