It looks like all you have to do is turn the knob on the top left of the camera to the A setting (the setting in the M, A, S, P modes), which should be aperture priority (I don't own the camera, but you can confirm with your owners manual), then compose your shot and adjust the aperture to whatever setting gives you the proper DOF. You have a depth-of-field preview button which, when pressed, will show you the DOF at the aperture you've set. Use a tripod and a remote release or the camera timer to trip the shutter. Pushing the shutter button with your finger will create movement, even on a tripod. Make sure you use manual focus. Autofocus is easily fooled in aquariums. Also, turn off all pumps so you have no water flow causing coral movement and no air bubbles. As was mentioned, small apertures result in slow shutter speeds and macro photography magnifies any movement, so you have to have everything as still as possible. It also helps if you set your ISO to 400, or even 800, until you get a better handle on getting good photos.
Gary