Well, I hate to contradict Bignose; I know he will come back with some published scientific paper refuting my experiences. I realize that a long-term blackout will deprive fish of necessary nutrients, especially vitamin D. I also know one of the tricks to getting half black angels to color up properly is in the light & dark periods in a 24 hour day. I think what is more in question is how much & what kind of light.
Here is a picture of the far end of my fishroom;
In the back, towards the left, there are a pair of 20's above a 55. The 20's are lit by a 40-watt incandescent bulb, that I will often keep dimmed half way during the day. They house a couple of breeding pairs of angels, who do really well. The furthest tank held a pair of apistogramma cacadouides about a year ago, they bred as well. It is commonly accepted that breeding fish are happy, healthy fish. The same 40w bulb lights the black 150-gallon tub you can see peeking in the pic on the right. The 55 on the bottom has an 18" single tube fixture, I've gotten plenty of pairs out of there in the past, it houses a group of half dollar to dollar size koi angels atm. In the foreground are some hatching tanks, little 2.5's.
Here's a pic of the middle of the fishroom;
Behind the 55 on the top is a glass block window, partially obscured by an exhaust fan & the vent it's attached to. Above the 55 is a 4-foot flouro fixture, single tube. I don't turn it on during the day; the window provided enough light for the 55, the 20 below it that houses another pair, and the hatching tanks. That middle rack is a busy area, tanks constantly changing. The very bottom has another pair of 20's, with two more breeding pairs of angels. Another 18" single tube flouro. Towards this end is another 40w incandescent that I only turn on in the evening.
That same 40w incandescent lights this end of the room's tanks;
The top row has a 2' single tube incandescent lengthwise at the base of the tanks on the far right hand side. The second row has the same sort of fixture, across the top of the three 29 gallon tanks. The 10 & 15 on the left get by fine with room light, and any light provided by the flouro fixtures. I do have a light on the bottom 10, it is so far removed from an overhead light source it does need it. There is no designated light above the second 150-gallon tub, ambient light does just fine.
Nothing I have is planted, so low wpg does not affect them. I've never totaled up how many wpg I have running in total, but the fish I sell do well for the shops & their customers, and the potential breeders I keep grow out just fine. I would probably have to get some sort of meter & figure out lumens, or some other scientific measurement, but no matter if it's a tank right under a flour fixture, or one off towards the end, the fish grow & thrive just the same.
As I stated, this is no scientific study, just my fishroom observations & me refusing to give any extra money to the utility companies.