I use an A550 so maybe I can help a little. I have 'live view' on my cam and yours should have it too, so as you change the light settings (tungsten, fluorescent ........) you should be able to see how it will effect the photo on the view finder. Tungsten will give you more of a blue tone, while the fluorescent settings lean toward yellow, adjust to your liking or change for effect.
Since fish rarely hold still for a photo op, HI ISO or a LO ISO w/flash is usually you best bet. I usually go with a lower ISO 80-200 with the flash. The flash tends to freeze everything in place and gives you foreground fill light. When shooting with the flash, shoot at an angle to the glass, straight on will give you a big white spot in the middle of the pic.
Use a tripod to steady the cam and use burst mode (cam continues to take pictures as long as the button is held down), you'll get 5-10 shots and 1 0r 2 will be good. NOTE: Burst mode does not work with the flash, because it can't recharge fast enough.
If you want close ups, use the MACRO setting (the flower button) for sharp pics. Macro works great for anything 3 feet and closer, which is where most aquarium shot will range. It works far better than trying to zoom from across the room.
Here is what I usually do: Try setting the camera up just far enough away so the that the aquarium fills the frame in the view finder with no zoom and at a slight angle. Depending on the size of the tank, you may still be able to use the macro setting (if you are within 3 feet) and shoot pics of the full tank with the flash and lowest ISO possible (80) is best, if the tank lights are bright enough turn off the flash and use burst mode. Then use a photo editor to cut the section of the pic you want to keep and/or post.
Doing it this way has many advantages; sometimes you get great shots you would miss if you were 'trying' to get them, photo editing on the computer renders much better results than using the software built into the camera, by shooting the whole tank sometimes you can get 2 or 3 great shots from a single picture.
Remember, the full resolution of the pic is 3000+ pixels X 2000+ pixels and you usually post pics in the 800 to 1200 pixel range. That breaks down to 3 full size photos in every frame without zooming or degrading quality. Using software on your computer it's easy to double the size from their with very little change in quality when you shoot the picture with no zoom.
And don't forget to shoot some video, I was surprised at the quality of the vid from mine.
Paint.net and IrfanView are two very simple and powerful FREE photo editors that you can download. If you really want to do some trick stuff try GIMP, it's free too and rivals any commercial photo editor you can buy.