Electronic pH Tester
> I am interested to buy one but, as normal, one my supplier says that is valid
I have owned the small pocket type electronic pH tester tester (similar to Coral Life) which I purchased from Edmund Scientific for 3 years and can attest to the accuracy and precision of the unit. As long as you periodically check and zero the device in a 7.00 buffer solution they should be accurate to 7.0 +/- 0.1 pH. Keep the glass electrode emersed in water.
The buffer solution last indefinitely if kept in the refrigerator. Any chemical supply company carries them, or if you have access to a University, the Quantitative Chemisitry Lab usually has Gallon jugs of the solution. About 2 oz. is all you need.