Black Moor turning gold/silver.

Hello,
I have a small Black Moor in a 10 gallon aquarium, right now he's about 1" not counting his rear fin. In the last week he has rapidly turned a gold/silver color over more then half his body, his head seems to be a light brown. From what I've read online this is fairly common in older fish, but seems to occur at a much slower pace. A week ago he was almost entirely black with a few shiny gold scales on his belly. Now he looks like this, also today I noticed part of his rear fin was missing a chunk.

Does any of this sound normal, I'm new to fish keeping and trying to do the right thing.

I'm using a Penguin Bio-Wheel Mini, it's been keeping ammonia at 0, I don't believe water quality is an issue even though I've been told my tank is too small for a goldfish(not sure I believe it, maybe when he's full size).

Thanks - Derrick

EDIT: I just checked the water parameters, I'm using an API Freshwater Master Test Kit(drops).

pH: 7.6
Ammonia: 0
Nitrite: 0
Nitrate: 80

Did a 50% water change due to the Nitrate, and added stress coat.

 

Hi Derrick

as you know fancy goldfish varieties originated from carp species
its very common every so often for these fancy species to develop characteristics of the fish that are their ancestors

I have seen common gold and other fancy varieties fish do this, start orange, or white or red etc and then change to the golden yellow colour of the original carp species they where developed from

Steve