Thats what I was thinking! I'm using salifert.
I was hoping you would say api... Salifert should be good. Try doing a small sample test just to make sure testkit works... take like a gallon of water into a bucket, add proportional amount of kno3 as you dosed before, mix and test. It's possible algae is eating up nitrates that fast, but I would hesitate to dose much much more right away before you make sure it all works.