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.
Wood tissue (witch in more detailed stans for shell, bone, and teeth)
True: light reactions occur inside of the the thylakoids membranes and they produce oxygen, ATP, and NADP
If those are the only answers i would say mutualism