When I take tank pics or even pics of fish, I make sure all the lights in the room are off with only the tank light on. If the tank light is bright enough you don't need the flash most of the time. When you do use the flash try putting a small bit of toilet paper over the flash to diffuse the flash back. If the room gets a lot of sunlight try closing the curtains or blinds as well as it can cause a lot of reflection on the glass to ruin the pic.
There are a couple of tricks that you can use to get better pictures.
Susan is totally correct that a dark room behind you will help a good deal. You'll want the most contrast that you can get between the bright inside of the tank and the dark outside. Try to keep the fish bright and yourself dark.
Adding a temporary light source in the tank will also help. I use a spring arm desk lamp over what ever it is that I'm shooting.
If you're really in a bind, you can try taking a black sheet and cutting a lens sized hole in the middle. Hang up the sheet in front of the tank and stick the lens through to take the pictures.
If you want to spend a little money on it, get a circular polarizing filter for your camera. It works just like polarized sunglasses by removing a lot of the glare/reflections from glass so you can see through it better, and it'll make the fish look more vibrant.
Good luck, I think you'll find it's loads of fun once you get started.
I turn off the lights in the room, and use my telephoto lens, so as to not spook the fish. I bump up the ISO setting so my shutter speed is fast enough to not induce motion blur. Keep in mind that as you bump the ISO, the photo gets more grainy with most cameras, especially compacts. It also helps if you have a lens that can focus when closer to the subject, so you can fill the frame with more of the fish.
Here's a shot of one of our Red Zebras, using the above tips:
This was taken using a Nikon D200 with a Nikon 70-300mm f/4.5-5.6G VR lens.
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