First off, you need to be in Macro mode(the little flower icon). That will allow the camera to focus correctly on really close objects. Most macro modes only work while not zoomed in, so you probably wont be able to zoom in much even in macro mode to get a close up shot. Most of the close up pics you see posted are crops of larger photo's, so what you will most likely end up doing is set the camera up for max resolution and max quality and take some pics, and then crop them on the computer. One other thing that will help with the quality is to hold the lens of the camera right up on the glass. This will help eliminate the glare from the glass and should help steady the camera also(not as important in you camera with IS). You can also use a magnifying glass and hold that up to the glass and then hold the camera up to that. If you have a problem with the coloration being too blue, or too yellow, you should set the white balance on the camera under your tank lighting, to get it to look good.
Hope that helps, you have a very nice camera which is very capable right out of the box.
cs
50gal cube in the works.
2x250w Coralvue eballast and reeflux bulbs for sale.