It's true that you need to have some nitrate but not more that 5 ppm
and phosphate of not more than .08 ppm
. It's just the right amount for photosynthetic corals to help them process their food. Having 0 readings or too much on both will deter the coral growth. There are some hobbyists with 0 readings on both but still having algae problem. You need to know first the things that make algae grow faster. Aside from those 2 mentioned their growth also depends on the kind of light, its duration and excess nutrients. You need to minimize them. On lights, i use more blue since algae loves the white (6500k). You also need to employ other means that can compete the consumption of nitrate before the unwanted algae takes it. Phosphate however comes directly from whatever you add in your tank like food and additives.