Ill fish?

Question
My grandad has a pet fish thats he's quite worried about. The thing is it was a golden god fish but its goldness seems to be fading and for the past week its been swiming sideways... If that makes sense. and hiding under the filter laying sideways - in which theres only about 1 cm and a half of space there. my grandad keeps pushing it out but it would swim back into the corner again as if it wants to be there.
We would be ever so greatful if you can explain why you think it's doing this. I've heard that gold fish live around ten years. So could the fish just be dying of old age?
Thank you very much.

And ai also appreciate it if you could explain why a black fish (my fish)would start to turn gold - on the belly, the lips and around the eyes.

Thank you

Answer
Hi Amal,

 Whell im not too sure but their is one think i can tell you is that if he is eating whell there is not much to worry about, and one thing that you should not do is push the fish, you may stress him and he may not eat.And about all that colore changing it is nothing to worry about, this is what some fisg do from time to time, whell i hope i have helped if you need any more info do not be shie to ask.THANKS!