If you have the ability to set a Custom White Balance, USE IT! The best way to do this is to use something completely white. I use a styrofoam picnic plate. Put it inside your tank, and take a photo of it. Try to completely fill the image with the plate and make sure it's very well focused. Save this photo, you'll need it.
Now, go to your custom white balance settings, it will ask you for a photo, to use, to set the custom white balance. Use the photo you took. This tells the camera "This is White." Now, the camera will automatically make the changes needed, to adjust for "white."
Our eyes are very capable of filtering out odd colors, to see true colors. In a room lit with fluorescents, we still see true color, instead of green. In our aquariums, we still see true color, instead of blue.
On the other hand, cameras are horrible at this, though higher end cameras are better. We have to tell our cameras what "white" looks like. The reason I say higher end cameras are better is this:
I used to shoot with a Rebel Xt and had to set a custom white balance for each of my aquariums, differently, or aquariums that I visited. I also had to set a custom white balance, when taking graduation photos, of my kids, in a gym, or they turned out yellow.
Now I shoot with a Canon 7D and haven't had to set a custom white balance one single time, taking many photos of many tanks, as well as photos in the same gym. It's "smarter," and better capable of interpreting "white."
