On both of my ryukin goldfish the tips of their fins are developing black on them. They were never black. My comet goldfish is completely unaffected and is the same dull orange though.
Responses (1)
This happens to most goldfish when they are introduced to a new tank. So you don't need to worry extremely much. However it might be because of ammonia poison (fish waste in other words) Fish turn black when the water is polluted to heal them selves. Having to said that, it could just be a colour change as they grow up and it'll eventually turn to it's original colour. This happened to my goldfish but it unfortunately died from fin rot. So this answer helped but I can't promise you that these answers will be in the case of your fish.