Use a tripod even when taking fish photos if you're not already. Even though you're moving the camera, the tripod helps reduce the shake you're going to get from hand holding the camera. Open up the aperture so you can increase the shutter speed. If you don't understand apertures, basically, you want a nice low f-stop (2.5-4.5). This will limit your depth of field (which may not always be acceptable), but it allows more light into the lens, which allows you to reduce the shutter speed. 1/100 shutter speed should be plenty to take pictures of most fish. The other way you can shoot at faster speeds is increasing the ISO (a.k.a. ASA). I'd go with 200 minimum, but you may need 400 or even 800 ISO to freeze your faster fish in frame. As you increase your ISO, your picture will become grainier, so play around with it a bit to get the right combination.