For a long period of time gold fish has been a pet for us due to its attractive colour and pigmentation. The real fact is that it should not be petted because the protective slime from their skin may be removed by touching and they became more vulnerable to infections caused by bacteria and Fungi. Another interesting fact about it is that it will lose its colour and pigmentation if you keep it in dark for a long period of time and may pale also.
When devoid of light goldfish skin stop producing pigments. Their skin contain chromatophores which reflect light to form colour of their skin and produce pigments for that. Actually the colour and pattern of goldfish is determined in the cells by the pigments as well as they are arranged in this pattern. The chromatophores . Other animals and reptiles also have this pigment called chromatophores to save themselves from their prey by changing colour and camouflaging themselves.
Thus darkness can result in paleness in case of goldfish to survive in the long run.