Sudden drop in nitrate

[Dinodanger]

I had 25ppm of nitrate two weeks ago now I have just tested and it’s reading zero? I have algae and cyano but it doesn’t look like enough to consume that amount of nitrate, a few days ago I added chaeto is it possible that chaeto can drain nutrients that quickly?

 

[lapin]

Not common but yes or you have a testing error or both.