It is virtually impossible to have zero PO4, a tank with no phosphates is a dead tank. Remember that our hobby level testing supplies do not have the resolution to detect that low. So what we see as zero is really not zero.
I'm registering 0.00 on my Hannah checker too. Some SPS have lightened up a bit, but most are just fine. I started feeding a little more, but not much. Color is returning and I'm still at 0.00. Unless you see a negative effect in your corals I wouldn't stress over it. My tank is fairly new and everything is just frags. I'm slowly adding more fish but not overly concerned because polyps are out and nothing looks stressed. If I remember correctly, the ocean water near reefs averages 0.003 phosphate. I realize that the ocean has a lot of available planktonic life that helps nourish the corals, but I'm not going to add a boat load of food just to register phosphates so that I can deal with green hair algae problems. If you need more color, just add some amino acids (sparingly) until you're happy with what you see.
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