Hi Micheley1101, I've modified your subject, I hope it makes more sense.
If you are shooting in an aquarium, then it'll normally be very dark. This means you'll need to use high ISO settings so that the camera can select sufficiently quick shutter speeds to freeze the action. Unfortunately the higher the ISO number, the worse the picture quality, but unless you can increase the light in the aquarium, you have no choice. Using flashes is not a good idea.
So I would put your camera into Normal mode, which lets you choose the ISO. Make sure the flash is also switched off. Try 400 ISO first, then if it it's still blurred, try 800, and so on. Of course you may find it's still too dark even at the highest ISO setting, in which case, forget taking photos and just enjoy looking at the fish!