I love white background! I used to have all black... Side, background, bottom, but once i changed to white, i have to admit, it makes your fish looks way better!
I was thinking black, but may change my mind if you show us a picture of your white background.
I seen videos of jobu23 750 gallon videos of his black diamond rays in blue background and man their white spots are amazing.. I think black background just make the rays even darker
Love his rays in there