My distance from the tank generally varies from right up against the glass to 3 feet away, depending on how much I'm trying to get in frame and how far back from the front glass the subject is.
My lens has a minimum distance to focus of about 6 inches. When I'm that close to a coral, about one square inch of the subject is in frame. When I was trying to get a shot of my entire peach Millepora (about 8 inches across?), I had to move about 3 feet back to get it all in frame.
Yes, I crop pretty much every image but the amount of cropping varies tremendously. Ideally the image I seek is perfectly composed in the viewfinder with every last megapixel of optical goodness devoted to the image. That happens about once a year for me. Usually I crop for aspect ratio. My camera shoots 1.5:1 in the mode I use and I often make 5x7 prints, so I crop the 1.5:1 original to 1.4:1 (5x7). A small amount of cropping.
Occasionally the only part of the captured image I want is a fraction of the original image. With a 15meg+ original image size, I can crop a lot out and still end up with a decent pic for monitor display though prints suffer tremendously. When I notice I've cropped out more than 1/2 or 2/3rds of the original image, I usually try to reshoot so that the part I want is doing a better job of filling the frame in the image capture.
One shot I took I noticed a postage stamp size area of the original image that I liked. After cropping I had too few pixels to work with, so I went back and shot a viewfinder full of just that area. Much better!
My usual suggestions include using a tripod (to hold camera still for longer exposure), a remote shutter release or timer (to keep from jiggling the camera while pressing the shutter release on the camera), shooting straight through the glass (to avoid clarity and sharpness ruining distortion) and focusing manually.
A common user error with point and shoots especially is getting too close to the subject or zooming in too much to the point where the camera is simply incapable of focusing on the subject. Simply taking one step back can resolve much of that. I'd rather see an image that's sharp but not incredibly close up than a close up blurry image.
Please also post a pic that's giving you trouble. The people here are very kind and usually quite helpful. While producing a stellar image can induce a wonderful feeling of pride and accomplishment, I find working with troublesome images helps me to learn more.
. I think the exposure will be a big learning curve with the new camera. I can already see the difference in the picture quality versus my old Olympus so I am still excited about that. I think I will hold off on any more pictures until my book comes and I get to read about Mr Blue and Green Jeans. I think that will help me get some good pictures of the tank 
. Currently I am attributing that to user error 
