I'll play a bit of devil's advocate here and say that the 100% water change may have served no purpose. This is because the phosphates and nitrates that have been feeding your algae problem may be tied up on your live rock. It may be a classic example of algae growing wild but test kits (which test the WATER) reading zero.
Changing out the sandbed was a good a idea, but you still may have algae issues from the live rock (esp. if it's as old as the sandbed was).
If algae issues reappear consider cooking the live rock while keeping the livestock in a QT.
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