Macros are fun. As a suggestion, play around with setting to find out what works. If you want more depth to your macro's, you might want a small aperture to get depth of field. But in low light, this may mean adding extra light and/or increasing the ISO setting. And then focus on the closest portion or most interesting part of the coral. Our you might have something in the background you might want out of focus. Increase the aperture so the back ground is blurred. A single focus point might be best here. If the coral is moving, you might want a fast shutter speed. To get this you might need to increase the aperture and/or the ISO. All would probably benefit from a tripod. The best thing about digital cameras is that you can take pictures with one feature selected and vary the other 2 and then compare photos almost instantly. I find I try to get the most light into the tank (w/o reflections), wear a black shirt, place the camera on a tripod, use a remote trigger, set the ISO fairly high, set the speed for moving or aperture for depth of field, aim the camera perpendicular to the glass and start shooting. And based on the first picture, adjust exposure compensation as needed. Experts probably just go into manual.
