Why did my small common goldfish turn a mottled black color?
A fish turning TO black isn’t always a good sign. The coloration indicates that it’s been exposed to some type of irritant or rough handling (think of this as being similar to a bruise or rash developing). Assuming you haven’t been abusing your fish, and there aren’t others in the tank bullying it and pushing it into the decor, I would first suspect your water quality. Goldfish produce a good amount of wastes, including ammonia, which is toxic to them. The ammonia is converted by bacteria to nitrite (which is also toxic) before being converted to nitrate, which they tolerate better. If you’ve only had the fish for a few months, or it’s in a small container (especially if it’s unfiltered), this may be your problem. Goldfish shouldn’t be in less than a 10 gallon tank as babies, and may need 10-20 gallons per fish as adults. Common goldfish can get up to 18 inches, and the volume is needed to dilute their wastes. You can take a water sample to your pet store for testing to see if this is
