You lose some PO4 - if it is biological uptake or if you are filling the chemical reserves (sand, rocks and so on) - I can´t say. But I have seen this before. Near zero - adding - still near zero - adding - still near zero - after sometimes - there is the ketchup effect - suddenly readings. You have to measure rather much until you hit the saturation point (if there is any). You must also count with the accuracy of the tester - its about 0.02 ppm (or around 5 ppb) - If you read 2 ppb - it can be from 0 to 7 ppb. If you read 0 ppb - it could be up to 5 ppb. If you read 5 ppb - it could be between 0 and 10 ppb. It is not easy in these low values
Sincerely Lasse
Yes, I see exactly what you mean about a potential "ketchup" effect where the numbers suddenly jump. I will keep an eye on that daily!
Thanks for the input! ;Happy