I have a small garden pond containing a small number of regular-variety goldfish. I understand that goldfish change colour as they mature but this morning I discovered that my large white fantail had turned bright gold/orange literally overnight! Yesterday it was white, today it is not! There are only 5 fish so I'm not mistaking it for another and it appears to be healthy. Is there an explanation for this? Apart from being a hot 43 degrees celsius here yesterday, the pond environment has been stable and not changed for several years. Thanks in advance.