| I use the 'poor man's' dimmer sytem on both my SW and FW tanks. By that I mean on my salty tank, my blue lights come on in the morning to simulate dawn for a couple of hours until the white lights come on and then vice versa at night to simulate dusk as well.
On my freshwater tank, I do the same thing, only I have two hood lights and one will go off an hour before the other and one will come on for an hour before the other to create basically the same things as in my SW tank.
I also use five little LED "moon lights" in my FW tank. I got them because I used to have some eels in this tank and they were very nocturnal. So, in order to see them I got the small lights.
When I switched to keeping cichlids, I went through the 'normal' growing pains of having to deal with aggressive fish and wanting to keep more than one at at time. When I figured it all out, I still had issues with the cichlids being really 'skittish' every morning. I eventually just stopped feeding in the morning on my way out to work and instead switched to feeding as I walked in the door.
I believe that I read in one of my magazines about creating a natural tank setting (biotope) and one of the things that caught my attention was how the author discribed that simulating dusk, dawn, and I believe he or she talked about creating 'dim' days to simulate storms or what not. So, I started to turn on only one light when I left for work and let my wife turn on the other as she took the kids to school about an hour and a half later. Eventually my fish didnt show as much 'shyness' in the mornings and so I went to timers (mostly for the weekends so I didnt have to wake up as early every day) and have used the system ever since.
From my understanding, this can be benificial for some fish and it couldnt matter less for others. I have also heard that doing this over a really long period of time, for example, only keeping one light on in a tank for a month and then slowly going to all lights on, can help to bring mostly nocturnal fish out during the day time as they just get used to the light. I havent tried that though and in my cichlid tank, I assume that it is better for my pleco to be nocturnal so that he doesnt get picked on as much.
So, I would think a dimmer is a good option for any tank. I have not bought one as of yet, but this is mostly because I spend my money on more tanks thus far and I have only upgraded things like filtration and decor and not so much lights other than bulbs. I have seen a lot of pretty cool gadgets out there though which will simulate the entire moon cycles and all kinds of stuff, but these are equal to buying a new computer so I stick to my good old trusty finger or a cheap timer to turn things on and off. | 
Dimmer 
