I have the same camera.
To the left of your display screen there is a button with WB above it.
Press and hold that button for a few seconds. You'll notice on the info screen near your shutter button, PRE is flashing.
Now, focus on a white spot in the tank, or place something white there and take a picture.
You'll notice the info screen on top of the camera will now flash GOOD.
Your white balance is now set for your tank.
Just be aware, different spots in your tank may turn out over or under balance do to light intensity. The closer you can get a white object to the area you are shooting the more true the camera with correct the picture.
Hope that helps. I've also noticed, pictures look great if you shoot in vivid mode. Tank lighting is intense so shooting in a negative exposure : -0.3 to -0.7 helps keep a photo from washing out.
If you want to change the exposure, hold the +/- button near the info screen at the top of camera, and rotate the dial on the back of the camera to increase and decrease exposure.