Those sticks on's are like a 50/50 shot... I have one in one of my tanks that I just never took off and it gives me a semi accurate reading. Its still off 5-7% but nothing extreme like yours. I would assume the accurite is giving the correct reading. I think I have like 15 accurites and 0 of them have problems. Place the unit about an inch or 2 off of the substrate to get the best reading as others have stated.