Studies show with acro growth a phosphate of .2 to .07ppm is optimum. The acro will be browner as a result of extra algae growing in it. Overall best levels for color is .07 to .05ppm is generally accepted. Below .05ppm coral algae start to suffer.
As with virtually every affordable test kit, zero isn't zero, it's just out of the range of that test kit. So a phosphate level of truly zero would be most noticeable as the corals algae starved.
I shoot to stay under .1ppm
http://www.nano-reef.com/topic/324253-maybe-low-nitrates-and-phosphates-arent-the-problem/