Pictures? Also, what's your fish load, water change schedule, lighting, and filtration?
PO4 is the obvious answer here. Are you using a Hanna or good quality kit for the PO4 and NO3? ATI are more or less useless when it comes to these two measurements.
Also, if you have dyno or aggressive algae, they will remove the PO4 very effectively so you get a false or zero reading on NO3 and PO4 tests. Assuming this isn't a very new tank, you must get NO3 and PO4 down to get rid of the aglae outbreaks. Seahares and even fish like a tang or foxface can fix the condition but not the cause.