As Hannah said, 0 is really what you want for phoshpate - and most test strips will always test negative, as phosphate is generally pulled out of the water by nuisance algae. In our systems there are really three things that are limiting to algae growth. The first is light, so if you're keeping a reef, nuisance algae will have all the light it ever needs. The second is nitrate - which is the by product of the final phase of the nitrogen cycle, and should be removed (at least partially) with weekly water changes. Ideally you want this as low as possible, but 0 is likely unrealistic - which means algae has it available for growth. The third limiting thing to algae growth is phosphate. This normally comes from tap water used for water changes, and fish food. Limiting phosphate is going to be one of the easier ways to give algae a hard time. You can do this be feeding flake and pellet foods sparingly, rinsing frozen foods before feeding, regular water changes, and possibly phosphate removal media. Algae is incredibly efficient at filtering phosphate out of the water, so most at home phosphate tests will test a false negative (meaning that in actuality, you have phosphates in your water)
