Size in goldfish translates into a neat bundle. Thus, it is wiser to be frugal and buy small goldfish 2-3 inches in size. Essentially, these colours are life long, getting affected primarily by ill health. Causative factors of include, small or poor habitat, unbalanced diet, and unclean water. At times, carotene can affect the colours of dark goldfish, giving them tinges of red.
Healthy goldfish are active, nosey and friendly.
In fact a healthy goldfish will come to the head of the tank eager to investigate as against thin, skinny and ill ones that stand on their head in a corner with clamped fins. Goldfish have characteristically symmetrical, even eyes. Though attractive these can be problematic in later years.