First lets sort out setting a custom WB. I know its tedious... but it will get easier I promise!
Setting a proper white balance is important because it will better your initial image. In most cases you wont even need to mess with digital color manipulation afterwards. If you decide the image still needs tweaked, altering colors later is not only more accurate but much much easier.
You can use any white material that you feel comfortable throwing in your tank. Frag plugs, PVC pipe, plastic sheet stock, paper, even your sand (not preferred but acceptable). The more 'white' your material is... and the more you can fill your camera frame... the better your results.