Algae problems with very low nitrate and phosphate levels
I have the nitrate and phosphate test kits from your range. Both consistently indicate 0.0 mg/l in spite of considerable algae growth. I also tested rainwater from outside that contained green "floating algae soup". 0 mg/l were indicated also in this case.
this phenomenon is not that unlikely at all and occurs rather frequently, especially in case of floating algae. The reason is quite simple: The algae literally take up the nutrients faster than they are being formed. They therefore serve as algae nutrients but are not detected by a test kit since they are not freely available in the water.
Too low nitrate and phosphate levels are not desirable at all, as this can actually lead to deficiency situations for the aquarium plants, which then do not grow well any more. Algae can, in return, benefit from this situation as they can adapt more quickly than the plants.
Dr. Bodo Schnell