0.5um Nitrate and phosphate is approx 0.03 - 0.04ppm (mg/L )
Numbers seem OK.
However - not being reef man by myself, so only guessing - if available kits go down to 1ppm only it either means that it impossible to go down further with this technology, or that there is no need for that in standard reef aquarium. I mean - if it is necessary, there is a market, if there is a market, sellers will show up from nowhere
Could be your aquarium is beyond standards, you will probably get couple of responses from others. I doubt in cheap solutions at this level.