Tom has taken most of the pictures on this web site, and at the time this show was recorded, he uses a Nikon D40. Here is his explanation of the settings he uses on this camera. These comments were not part of the show and are added here as more detailed information.
 
When I bought a Nikon D40 camera, it came with a 18-55mm zoom lens, which is very widely acclaimed for its high quality and low price. It's a bargain! The Nikon D40 is also a big bargain. The combination of this camera and lens is highly recommended.
 
When I took my Nikon D40 out of the box for the first time, the mode-dial was set to Auto. The mode-dial is on the top-right side of the D40. When the mode-dial is set to Auto and the lens is set to A for Auto-focus, the Nikon D40 almost always takes a great picture of normal scenes without any adjustments to anything else on the camera. But these automatic settings rarely take good pictures of live fish in aquariums.
 
Note added later. The Nikon D40 has been replaced by the Nikon D3100, which may be slightly better for photographing fish in aquariums. Nikon also now produces the Nikon D7000, which is even better but sells for substantially more money.
 
 
Set Your Camera to Manual.
My first hint to take pictures of fish in aquariums is to rotate the mode-dial from Auto to M. M is for Manual, which will allow you to set both the aperture and the speed to any value allowed by the Nikon D40, including some values that will produce very poor results, but we are hoping for a combination of settings that will produce better results than the Auto settings. 
 
Adjust the speed and aperture settings on the Nikon D40 as follows:
 
First, press on the Info-button until the speed/time and the aperture show on the Nikon D40’s LED-screen.
 
On the upper-right side of the Nikon D40 is a horizontal black dial, called the command-dial. Rotate that dial until 500 appears, which  means 1/500 of a second. This is the fastest speed that syncs with the flash.
 
Next hold down the +/- button and rotate the same command-dial to adjust the aperture to F 8, which is often written f/8 but appears as F 8 on the Nikon D40’s LED screen.
 
You should probably also adjust the ISO. I start with 400. I adjust it by holding down the Fn-button, and rotating the same command dial.
 
Press the button near the flash to pop-up the flash.
 
I set the focal length on the zoom lens to between 35 and 40mm, but I may change this to a different focal length later. I usually take the final pictures with a focal length somewhere between 30 and 55. How do I decide? By looking at the LED or sometimes by viewing the pictures on a computer, adjusting the focal length, then shooting some more pictures.
 
I almost always use the Nikon D40’s auto-focus, which seems to work very well for me with the 18-55mm zoom lens. I have tried several other much more expensive lens, and the auto-focus on those lens for a variety of reasons did not work as well for me, when taking pictures of aquarium fish!
 
Practice Taking Pictures.
If you get reflections, you are aiming the camera at an angle that is too perpendicular to the front surface of the aquarium.  Aim the camera at a slight angle. Vary the angle, and then look at the LED for reflections in the picture you've just taken. Continue to vary the angle and look at the result in the LED, until the reflections are gone. I had to practice doing this for a while, but now it's second nature to me.
 
Next you need to deal with the exposure and depth of field. If the images on your LED are too dark move the f-stop to a lower number like F 5.6.  If the images are too light, move the f-stop to a higher number like F 12.
 
If part of the image is in focus but other parts are not, you may need to increase the f-stop, which will increase the so-called depth of field and bring more of the fish into focus. But an image at an f-stop of about F 12 will begin to degrade due to diffraction, so I try to stay between F 8 and F 12. At f5.6 the depth of field is thin and parts of the fish may be out of focus. Bigger f-stop numbers create a deeper depth of field and can be useful. But then you get less light and may need to increase ISO. I usually end up at about F 10 at an ISO of 400.
 
If the image is too dark, increase the starting ISO from 400 to 800 or even higher. If the image is too light you might reduce the ISO from 400 to 200.
 
Remember to have Fun!
It's lots of fun to vary these setting one by one and see the effects in the LCD. All of the pictures, shown above, were taken using the methods described on this page. But it seems to have taken me quite a while to find a combination of these settings that produces a picture that I like, and each new fish is a new adventure, which requires a new combination of settings to get a picture that I like.
 
I hope these tips help some of you to take better pictures of your pet fish.