So, to get started, since you have a tripod, initially, let's not worry about the shutter speed. Set your camera to Av mode, with ISO set to 100.
Now, set your aperture to F/8.0.
Chances are, the Av mode of the camera will determine that your shutter speed will be anywhere from 1/4 - 3 seconds. Take the picture, how does it look?
Is the tank overexposed? If so, you should have exposure compensation, which Jeff mentioned. On Canon, that's usually indicated by a series of lines on the screen, like:
|--|--0--|--|
If it's too bright, move it two bars to the left of 0, and shoot again. Better? Fiddle some more.
Now, the tank might look great, but your fish are probably blurry, right? Maybe even your plants if you forgot to turn off all of the flow in your tank.
Now, you have to get the shutter speed down to some less than 1/40, preferably 1/100, but that might be impossible with your camera. (Side note, I doubt any lenses for your camera will make a difference for this.)
So, to get the shutter speed down, you have a couple of options:
1. Add more light over top of your tank.
2. Increase your ISO. The more you do this, the grainier the resulting image will be. You'll likely have to just deal with some graininess.
3. Decrease your aperture. The lower this goes, the more limited your depth of field. This means, that the lower you go, the more likely you are to have the things in the front of the tank in focus, while the things in the back, may not be so focused.
In general, if you can't add more light, you'll likely have to compromise on one of both of these. I would start by pushing the ISO to 400. Adjust as necessary from there.
Report back.
