Cardinal Tetras Losing Color

Seemingly overnight, my entire school of 8 cardinal tetras have lost almost all of the red on their bellies...they're practically white on the bottom now. They're also lethargic, seeming barely able to swim, and they're not schooling like they normally do. My water parameters are 0 ammonia, 0 nitrite, 20 nitrates. What could be the problem? My other fish seem fine.

 

Could you post a pic?

 

It's difficult because my phone camera sucks.

They seem to be returning to normal. I was away all of today and just got home and turned the light on half an hour ago, and was alarmed at how lethargic and white they were. After feeding and some time with the light on, they seem okay-ish. Could that be the issue?

 

Yes, that might be it, since fish are "sleepy" and don't have as much color after the lights are turned on.

 

Their bellies went from white to bright red after feeding. And all the ones that were sitting in the gravel are now schooling again.

 

They lose their color when the lights go out and they "sleep". Your nitrates are higher than their liking though, Cardinals are a bit sensitive, you should aI'm to keep your nitrates 10 or less with them. Other than that, yeah its probably just the light. When I had mine they would be pale as ghosts in the morning when I first turned the lights on and very slow moving, then as the lights were on theyd color up and start moving around, completely normal with the species.

 

That's good to hear....

 

Thanks. I had a rough day today, then I got home, turned on the fish light and thought my fish were on the outs. Hopefully, that's not the case.

 

Well, good luck and hope they are OK.
I think they'll be fine and nothing to worry about. Sad to hear you had a bad day....

 

May tomorrow be a better day! TGIF right? Dont stress about your Cardinals, like I said its completely normal. They almost look dead every morning, and then magically come back to life. In the wild, they are very low on the food chain, so part of this is to make sure they don't become late night/early morning snacks, they grey out and become very still, mimicking dead fish so they look less appetizing as most wild fish like Angelfish that hunt them in the wild will prefer live prey.