Lighting for indoor tanks

I have fish in quarantine: they're in a black tank in a shaded corner of a
covered porch. In other words, it's pretty dark. They're acting lethargic,
but will move around if a flashlight is shown on them. I don't think they're
sick - I think *they* think it's just night-time so they're being quiet. Do
you think adding a light over the tank to simulate "day time" would make
them act more normal? Those of you who overwinter your fish are in a much
better position to answer this than I am.

Thanks!

Lee