Goldfish Fins Healing. One of the most dangerous causes of goldfish scales turning black or goldfish fins turning black is that your fish are suffering from ammonia poisoning. If fish are paired with aggressive or incompatible species, they will most likely develop fin rot.

Once the goldfish are dipped, the fins and slime coat get very ratty looking. High concentrations of ammonia and nitrates in aquarium water can inhibit fin healing and regrowth. The goldfish will look normal in.