depends on the flourecent lights.. . . I know for standard ones it cannot be done, but they do sell light strips (low wattage flourescents) that you can dim.
I was also told they can not be dimmed, since they are a gas that lights up and not a filament. If you have more than one plug per bulb than you can have them alternate to turn off. Also I have lunar lights which turn on when the main lights go off.
Certain compact fluorescents are dimmable, but cost (at least when I bought a bulb) almost twice as much per bulb. I use one as part of my motion-detecting front porch light.
With respect to aquariums, check this out (if you can afford it):