I'm not familiar with Nikon in particular. But normally, you can set the white balance on the camera. You could have a few go's with these settings if you want.
The other possibility would be to use a grey card. This is a card with a specific grey composition. You put it in your tank (make sure it is safe) and then take a photo. Your software then will be able to get an idea how to compensate to make that grey truly look the way it should. Depending on your lights, this correction may be different in different places of your tank.
In the end you will not get around of playing with raw. I think, from my limited experience, that the lights in our tanks throw off the auto white balance.
What always helps is to reduce the amount of blue light for taking pictures, if you can.