So yesterday I bought a blue guppy. He is a male that is completely blue with a yellow tail. During the night, I turn off the light in my tank. But when I woke up this morning and turned on the light, I noticed he was completely pale and white. When I turned on the tank light, he regained his color after a couple of minute. Why did he lose his color? What (if anything) is wrong with my guppy? Is this normal for guppies? Is there any way I can prevent it? Please help...
Getting moved to a different aquarium is quite a stressful experience for a fish. Next time, you may want to try drip acclimation:
Fish do have circadian rhythms, and they do sleep. Most mornings I turn on a room light at least 30 minutes before turning on aquarium lights. Suddenly going from total darkness to bright light sometimes scares fish, and this along with the moving stress likely caused the color fade.