Q. We bought a black moor goldfish that was all black. After a year the black moor developed beautiful red markings on its fins and tail. Now the red has disappeared and there are narrow white stripes along the fin edges. Are these color changes common in black moor goldfish?
A. Black moor goldfish do tend to change color over time. The black coloring in black moor goldfish is very sensitive to water conditions and lighting. Any significant change in water conditions — pH, hardness, dissolved oxygen and so on — can produce a color change. Intense light increases the black coloring in black moor goldfish. Low light conditions result in bronze or red colors peeking through. White lines in black moor goldfish usually develop with age.
You may try keeping the black moor goldfish in an aquarium that receives lots of direct sunlight, which tends to make the black coloration deeper and more velvety.