well common things, Lights are said to aid in fast growth, so cut down on lighting if there on longer then 8-10 hours for daylight and what not.
More flow helps suppend material in the column longer to be removed by the filter.
What kind of filter are you using.
My guess is most of the parameters are spot on because of the algae consuming the nutrients resulting in a false negative.
Also how much do you disturb your sand bed. If a lot, stop lol, because releasing all that junk into the column just stirs of phosphate and nitrates and can take a while to stabalize again.
Water change, how often and how much
what type of water, RO, well, city, bottled, etc??
what kind of test kit are you using. I find that salifert is usually very accurate over like API and even elos.
just a few things to help us help you. Cyanobacteria (red algae) is a very fast growing "algae" that is a sign of poor nutrient export usually. So my best advise is to keep up on the water changes, Watch how much you feed, as frozen food, if its used, has a ton of phosphate saturated in the food, when thawed, can release and then be put in your tank. its best to rinse in regular water, drain and then feed to help eliminate all of that before feeding. As small as that sounds, dailey or even every other day feedings can be enough to throw things out of wack.
one last thing, are you using a skimmer??