I've been checking my phosphate with an API test kit and always measures zero. I took some water to my LFS and they came up with .11. From my readings, .05 seems to be the high limit of where I should be. I need to be able to test this more accurately. What is the preferred tool for checking this? I've seen a lot of people post about a Hanna but I don't know which is the best. There seems to be several models. Any help would be appreciated. Thanks!