Great questions. I had 0 nitrates because I was skimming heavily and I wasn't feeding as much and I clean my filter socks religiously. I've upped my feeding to 6x / day and that seems to have helped. Fish look happier too. I don't have to dose nitrates as much anymore but it does drop.
Bioload is yellow tang, hippo tang, velvet wrasse, 2 clowns, 3 chromis and a midas blenny.
I always test 0 phosphates but I see algae growth and have to clean glass every few days. I use salifert and it's always 0. It's hard to tell but one of the guys at Neptune said to use two vials and have one with only tank water and compare them side by side, and I do that and they vials look the same. I only dose phosphates when my zoa skirts look a little thin. I'll .03 to .05ppm and by end of day it will be 0 again.
I have dealt with cyano when I ran my fuge for 12 hours / day, which is why I started dosing phosphate and nitrate (and the colors/growth not being so good). Now I only turn on my fuge light every few days for 6 hours or so after heavy feeding.