I decided to mess around tonight with depth of field (DOF) with shots of my favorite coral. All were taken with my Canon 7D, EF 50mm f/1.4 lens at 1600ISO. I was about 4 feet from the coral. This one is at f/1.4 and 1/5000 second shutter speed Notice the narrow plane of focus and how the in focus area stands out from the scene. This is a good way to isolate a subject and bring it to everyones attention. This one is at f/4.5 and 1/500 second shutter speed You can see more is in focus but not all of the coral. You will notice the in focus areas stand out but not as noticeably. This one is at f/8 and 1/160 sec shutter speed On this one you see almost all of the coral is in focus and that it does not stand out much at all. Your DOF will vary depending on the focal length and distance from your subject. The closer you are the less DOF and conversely the farther away the greater the DOF. Shorter focal lengths give a deeper DOF and longer focal lengths a narrower one. My camera is a crop sensor camera and has a deeper DOF than a full frame camera at the same settings. Most mirrorless and point and shoot cameras have even smaller sensor than crop sensor cameras causing their DOF to be even deeper at the same settings. Here is a good site to get an idea of what your DOF will be with your camera, lenses, f stop settings and distance from subject. Experiment and have fun taking pictures of your tank and its inhabitants.