pH and Tap water

I point out this bit constantly online, don’t feel bad if you didn’t know it, I didn’t understand it in the beginning either and really got frustrated with hydroponics because of it.

Anyway, when you fill a container from the faucet or the hose with water and immediately test the pH, you will probably get a false low pH reading.
WHY?
Because of trapped CO2 in the water. Dissolved CO2 acts as a weak acid (carbonic acid I believe.) Once this CO2 has time to off gas (you can aerate it out like you would with chlorine but this effect with the CO2 happens in well water as well as in city water) the real pH of the water can be seen.

Back when I was doing hydroponics I had a terrible time because I didn’t know about this and no on on the Hydroponic forums I was on shared the info. Anyway, I would test my tap water or well water pH and it would read 7. Well that seemed pretty good since I knew my nutrient mix would lower the pH even more so cool. Then I would mix it all up and get things going and before I knew it, the pH would be up way over 7????? What the heck? I didn’t get it.

I will take a moment to caution here about the use of pH meters too. They are not always reliable and need to be checked, calibrated, cleaned, maintained and stored properly. This usually takes solutions for each of these steps that you have to buy separately so please don’t assume that a pH meter will save you money in the long run because you don’t have to keep buying the test strips or test solutions since you are simply trading the test solutions for calibration and storage and buffer solutions instead as well as replacement batteries and replacement elements for the meter. If something seems off with the pH meter, you should have an additional means of checking pH so you can compare. I had a pH meter fail but I kept trusting it and adding more pH down and more pH down and more and more before I realized that the meter had quit working by which point the nutrient mix was trash with a pH below 4 and I had wasted most of the bottle of pH down. Now that incident was back in my days of hydroponics.

I will say that had that incident occurred in an aquaponics system, it would likely have killed fish and the bio-filter bacteria as well. If something seems off, stop double check things and ask for help, there are online forums with plenty of people willing to offer advice and perhaps tip you to an idea or issue you hadn’t thought of.