I have the same problem and have for over a year. I rub carbon and GFO some times and have macro algae in the sump. My Hannah checker always reads 0.00. I did get one reading once of .03 which is my target. I used to feed a small amount of LRS reef frenzy every other day with reef chilli every other day. My corals were losing color and growing slowly. I kept losing zoas slowly too. For the last month, I have increased my feedings by doing double the amount of both reef chilli and LRS reef frenzy and feeding daily now. I have the apex auto feeder dosing pellets once a day too. I also started adding Marine Snow and Red Sea Reef energy A and B. Since starting this new regimen, my phosphates are still 0.00 but my corals have all colored back up and are getting fat and happy. I have reduced the usage of the carbon and GFO but I still use it some. I also thinbout my macro algae more often as it grows like crazy. I also began dosing small amounts of Kent's Iron/Manganese supplement. Algae uses up these elements and they are needed for the algae in the corals too. Unfortunately, I have a ton of algae in the display and have always had a hard time keeping it at bay. It grows even better now so I just added more clean up crew. The corals and fish are much happier and healthier. It took about 4 weeks for things to start looking better.
Sorry for for such a long answer but in summary, try Marine snow, and Red Sea reef energy and feed more in general.