Hi,
I just bought a Nikon D40X, and don't know how to use it very well, either. Do you have a separate flash when you shoot in the tank, or do you use the pop up flash? I bought a series of macro lens filters that thread onto the front of the lens, and I'm assuming that's what you used, too. One thing that I did learn, to my dismay after I had already bought the camera, is that if you want to use different lenses and want the autofocus, you've got to spend the extra money to buy the AF-S lenses. Just any ole Nikon lens mount won't work. BTW, I love the shot of the big coral with a completely black background, very artistic!
Joy