Goldfish are one of the easiest fish to care for in the home aquarium, and they look really decorative in their wide variety of colors, including orange, silver, white and black. Over time, these colors can change, usually due to conditions in the aquarium.
Natural Color Change
Goldfish colors can change in response the color of their surroundings. They have different types of cell in their skin and some of them produce melanin, a black pigment. A dark environment, such as a tank in front of a dark background or with lots of dark rocks, can cause the fish to produce more more melanin. This shows up in fins or patches on his body.
Ammonia
Ammonia is a byproduct of rotting food and fish waste in the tank, and it can affect a goldfish's skin. Ammonia burns themselves don't show up but, when the ammonia levels drop, his skin heals and affected patches turn black. The black patches can appear on his fins or elsewhere on his body. Constant high ammonia levels don't give his skin time to repair itself, so black patches may not appear at all. Try not to give your fish more food than he can eat and change his water regularly.
Disease
It's unusual but goldfish can turn black due to disease. Water snails carry a disease called black spot. Eggs of a parasite burrow into fish skin and protect themselves by forming a hard, dark cyst. This causes dark patches on the fish skin. The disease doesn't affect adult fish badly, though they may show irritation by flicking their bodies. Removing snails from the aquarium breaks the parasite's life cycle.
Goldfish Care
There are lots of attractive fish tanks available. Buy the biggest you can afford. The bigger the surface area, the more oxygen available to your fish. Goldfish live best in stable water temperatures, so don't stand your tank in direct sunlight. Every three or four weeks, replace approximately one-quarter to one-third of the tank water. To allow chlorine to evaporate and the temperature to reach room level, let fresh water stand for three or four days before adding it.
