I did the same thing in a 90g and have been keeping some cuprisorb in but it is no longer turning blue. I think the Hanna checker is off for low range as others have said! In the process of removing copper from a 220g. I just wanted to nuke the ich and velvet after losing $1,000 in fish.
I was in the same boat. Assuming you don't have inverts in the tank, it's good to have this info in your back pocket. I think this info is useful for those who don't have the means to quarantine a large amount of fish.