Young Changing Colour From Black to Gold
We have a reasonable number of gold fish in our garden pond. This is about 6 ft by 4 ft. and 2.5 ft deep in the middle. It is about 15 years old. We have a filter and a waterfall.
We bought fish originally - just small ones. Alas the local heron found them good for starters and so when they reached a fair size about 7 inches, they were snapped up. We found in the next spring that there were very small fish in the pond, obviously born in the pond but they were black!
They grew and grew and eventually turned gold at which point, the heron was back for the next course and this sequence has gone on since and we have a reasonable "fleet" of mixed breed Koi and fantails.
Is the change of colour to do with their environment as when I bought very small fish they were all gold/gold and white or tri-coloured.
The black colouration of your Goldfish has nothing to do with environment, but everything to do with the process of turning orange/gold.
Your fish are metallic scaled, in other words, they have a bright shiny metallic sheen.
All metallic Goldfish start off silver gray, turn black, then slowly fade to a light yellow before their colour deepens to orange or red, sometimes with patches of white.
Metallic Goldfish can start changing colour within 3 months of hatching.
Goldfish change colour at different rates. I suspect the heron has picked off the faster colouring individuals when they were small, and when the slower colouring fish, which had continued to grow, changed colour they were then an easy target.
If this cycle continues, your surviving fish will take longer and longer to change colour as only the slower colouring fish will reach maturity.
I would suggest covering the pond in early spring one season and selecting the fastest colouring fish to grow on away from the heron.