What do you know about photography? I'll assume not much, so apologise if this sounds patronising...
To start with try using Aperture Priority (Av on your dial) and set the ISO setting to Auto. Also set the White Balance to Auto (AWB). Shooting in fine JPEG is a good start too with minimal need for Post Processing,
This way you control the Depth of Field (how much remains in focus from the foreground to the background).
The camera will automatically control shutter speed, ISO and white balance for you, leaving you to concentrate on composition and focusing. Experiment with your auto focus and manual focus.
If the shots are over or underexposed (too bright or dark) then dial in some Exposure Compensation. I'm not sure how this is done on the xxxD range, sorry.
If your fish shots are blurred then you'll need to use a faster shutter speed. Try using Shutter Priority (Tv) and manually select a fast shutter speed i.e. 1/100th or faster.
That's some very basics to get you started.
I recommend reading up on basic photography and the relationship between aperture, shutter speed and ISO. Once you've grasped that concept you're free to concentrate composition and what consitutes a good photo.
Here's one of many tutorials, but I like this forum -
http://photography-on-the.net/forum/sho ... p?t=414088
When you're confident with your camera and its settings you may wish to shoot in Manual where you control everything.
Have fun! Aquarium photography is regarded as one of the toughest styles, even by the pros, so don't give up and practice, practice, practice!!