You may have a few good pics. To freeze the motion of swimming fish(anything moving infact) you need a quick shutter speed. The more light on the subject the quicker the shutterspeed and vice versa. You can get some good motion effects with less light but the pic won't be as sharp. Try taking a couple of pics with the falsh,without, camera close to the tank and the camera at an angle to the tank, if your camera allows you to adjust the shutterspeed try changing that as well a couple of times. To some it may seem like a waste of film but you will see how different things and settings affect the quality of the pics, and you can then figure out what gives you the best reults.
If you've got a tripod it's best to use one as it will reduce camera shake. If you haven't got one try the arm of a chair or step ladders, with a small beanbag or cushion for the extra suport. you can always use a rolled up t-shirt etc.
Mostly take notes! it will help at some point in the future. As I use an S.L.R. camera I write down everything (f~stop,shutter speed, light used, exposure time,etc) then when I check the prints I can see what all the settings were for the best quality and use them again. I'll do some checking into the type of camera and see if I can come up with some specific tips for you.