I think it depends on what fish your keeping and what colors you want them to exhibit. I have straight black sand in my 200 gallon with my frontosa, and black floramax and white sand mixed in my planted 55.
If you have your heart set on white. Just go buy the sand, and be sure to rinse it before putting it in your tank. Scoop out all of your old gravel, and slowly add the sand. Be sure to turn off your filter while you do it. Floating sand particles can mess up your filter.
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