There is no mirror. It's an electronic VF according to Canon's website.
If your tank lights lean green then use the fluorescent wb. Works most of the time since fluorescent lights generally leave a green cast.
OK a couple of tips for the beginner.
1st Set your metering to Center Spot if your camera allows it. Most are set to averaging or center weighted.
2nd Set your focus point if your using auto focus to Center if your camera allows it.
Interesting, so like an LCD screen then?
What benefits are there to Center Spot Metering?
I will be putting a section in about how to use auto-focus or manual focus and get the results you want, and using the light meter in your viewfinder and the benefits or reasons behind using Center Metering would be good info to have.
I'm not highly confident that I know the real benefits - however I do have mine set to center metering just so that I can point the center on some sort of neutral green in the tank and balance my light meter based on that.
In black and white photography I learned that your light meter only sees the color grey - and no matter what it is looking at it assumes the exposure is correct when whatever you are pointing at is the same tone as 18% grey.
That might not be completely correct, but thats how I remember it and is roughly correct =]