Blur comes about through two reasons. Either the subject isn't in focus or too slow a shutter speed has been used. To ensure that the camera doesn't choose a low shutter speed you need lots and lots of light. (Alternatively you can force the camera to use a fast shutter speed but you'll then have a very limited depth of field which, with the fish swimming towards and away from the lens, will be difficult to handle and probably produce 'out of focus' results).
So, initially at least, forget about the camera settings and work out how you can maximise the light in the tank. If it's a very small tank (or you can temporarily move the fish into such a tank) you can move it close to a window or, far better still, actually take it outdoors. Otherwise take your pictures on a bright, sunny day, with the curtains in the room open as wide as they'll go. Then try to add to the light by pointing every available desk lamp at the tank. (That might slightly throw the colour balance out but Photoshop could probably correct it with just a couple of clicks anyway).
With all that external light you'll need to avoid reflections off the glass, so you'll need to get the lens of your camera right up to it. (That should also prevent your camera from trying to focus on the glass, giving you a better chance of being able to focus on the fish).
Unless you're very lucky (with the fish staying still just at the right moment), you won't get any decent pictures with a shutter speed slower than 1/100 of a second but 1/500 would be a better speed to aim for.
The foregoing, of course, assumes that you've not got suitable flash equipment available, as that would allow you to maximise the light and freeze the action.
Those are just my initial thoughts. This offers far more detail:
http://www.tfhmagazine.com/details/articles/fish-photography-secrets-full-article.htm