Use a tripod, your movement will make the picture come out blurry unless you can shoot at a fast enough shutter speed... and you probably can't unless you're using a flash.
Turn off all of the lights except for the light in the tank, the other lights will just cause reflections and you won't spook the fish or attract as much interest if you're "hiding" in the dark
Use higher speed film or bumb up the ISO on your digital camera, you'll have to decide if you can live with the noise it creates, but it will help you to use a faster shutter speed. You want a fast shutter speed so your fish don't have as much time to move and blur the shot
You can try using a larger aperture when you take the shot, shooting at f/4 will produce less depth of field than f/8 but once again it will allow you to increase your shutter speed
Be patient, once the fish get used to your presence they shouldn't be as frightened. The others will eventually stop begging for food and getting in the way of your shot, if that's the case.
Try try and try again. If you're using digital then take a TON of pictures, the more you take the more likely you are to have a few successful shots.
Keep the camera parallel to the glass. If you shoot through the tank at an angle to the glass it will create some distortion. This becomes painfully evident if you ever try shooting your tank with a long macro lens. In the case of a bowfront it will be harder to get the camera parallel but just try your best, the disortion isn't as noticable with regular/wide angle lenses
try some tricks to get your fish to hold still, maybe you could turn out the lights for a few minutes until they stop moving... focus on the fish and turn the lights on, if they are the kind that take a minute or so to become active then you'll increase your chances of getting a shot without as much blur
if your tank has high lighting it will make your job easier, the brighter light will allow you to use a faster shutter speed
if you were shooting perpendicular to the glass then yes you'd probably be facing the wall, what you want to do is keep the front of the lens parallel to the glass. like this ||
A few more tips I have heard. Set the camera up on a tripod and leave it in the same spot for a few days. The fish get used to it and don't pay any attention to it. Beside that and whats already been said, take hundreds of pictures you will get some good ones but most won't be. It happens to everyone as far as I know.