I wouldn't waste all that time testing four times a day and fretting over this kind of issue. Test it once daily and if the level drops close to the bottom of the range you've identified for treating fish just add some copper and recheck it later in the day.
Your original goal is to treat fish not to see a specific number on a meter. As long as you are in the right range the main goal of treating fish is being met
If your main goal is to see a number on a meter that you like then you should call Hanna Instruments and discuss this with their customer support to see if they can identify an answer.
Your original goal is to treat fish not to see a specific number on a meter. As long as you are in the right range the main goal of treating fish is being met
If your main goal is to see a number on a meter that you like then you should call Hanna Instruments and discuss this with their customer support to see if they can identify an answer.