Using a regular pH tester like Apera PH20 and PH60 to test pure water like tap water, drinking water, RO water, distilled, and deionized water could be a challenge. It will typically take much longer for the readings to get fully stabilized (typically 1-5 minutes). Please be patient. 

If it's not working, try adding Apera 3M KCL (AI1107) to your pure water at the ratio of 1:1000 (e.g. 1 ml KCL to 1000 ml water) to accelerate stabilization while minimizing pH change. 

If the accuracy is not meeting your requirement, a pH meter with a specialized pH electrode designed for pure water test such as PH850-PW or PH8500-PW would be the better choice.