I'm just getting into sps and learning the ins and outs of them. My phos had been reading around .08. I'd taken my algae reactor offline and over about a month period the phos crept up to .13 and I noticed my bonsai went from a nice dark purple to a very light purple. The phos was the only thing that had changed. I added some chaeto to my fuge and the phos slowly came back down and it colored back up. For me I think that's the sweet spot. My nitrates are somewhere between 5-10...I can never tell exactly with trying to match the colors on the chart!