Fish can respond to gravel color within an hours time. The fish store would probably love to sell you a bunch of T5 HO bulbs though, they will probably even try to get you to buy a block of frozen cyclopeeze to boost their coloration. Bottom line you have plenty of light, and 6500k is a good spectrum. Luckily some black blasting sand is really cheap.
I might as well ask... What species of fish did you buy a week ago?