It is probably absorption. Do you have media in the tank or in the HOBs?
I use Furan-2 all the time in copper. I always test right before the dose and right after. My results are always within the +/-.05ppm tolerance of the tester.
Just a foam pad in the HOBs. But that’s interesting. I’ll keep testing when I can this week but I suppose it’s time my better 3/4s learns how to test for copper [emoji51]