Here's my best guess. There are a number of things going on here.
First, as others pointed out, your focus is probably off. Someone mentioned minimum focusing distance. You may be too close for your camera to focus. Also, camera's autofocus systems have difficulty in low light, so you might want to increase light in the scene (while avoid glare) and/or use manual focus.
Color: You said that your fish is not that gold. I believe two things are contributing to that. First, your white balance may be off. Also, the image looks underexposed. Underexposure sometimes appears to saturate and deepen color. Increasing exposure might make the fish look less bronze.
Motion: Even if you fix the above, you may still be presented with the problem of motion...motion of the fish and motion of the camera (if you're hand-holding). You should find a time when the fish doesn't move much. For example, if he/she gets active around meal time, avoid shooting that that time. Some animals react to light (flashlight), etc.., so consider what you can do to get the fish to stand still. If the fish moves you're going to need a shutter speed that will freeze his motion. That will require more light, and/or a higher ISO, and/or a wider aperture (which could make focusing difficult because of shallow depth of field). If the fish is reasonably stationary within the tank, you could stabilize the camera on a tripod (or be creative and use some other means to stabilizing it). Finally, you may just need to be patient. Instead of chasing the fish around the tank, set up your tripod and your focus and wait for the fish to return to this zone. With this method you may need to use a narrower aperture to get a little more depth of field so that you have a larger area of focus.
Good luck!
