Why does the pH keep rising?

This is really just a reminder about a blog post from ages ago.
http://www.aquaponiclynx.com/ph-and-tap-water-2

See I just read an aquaponics newsletter that made me think people will need to know this tidbit or they could find themselves repeatedly washing their concrete tanks with acid for years and still have a higher pH a few days after filling the tank with their tap water for ever even if the concrete tank had stopped leaching after the fist acid wash.

See many of us doing aquaponics have hard water but if that water is coming from a city water system or from a well, it probably has carbon dioxide dissolved in it which is acting like a weak acid and if you measure the pH of that water right out of the tap, you will get a false low pH reading. Let that water air out for a day and you could have a drastically different picture of what your water is like.

See I could fill a plastic tank with my well water and check the pH of that water right from the hose and get a pH of 7. And then tomorrow I could measure the pH of the water in the tank and it will probably be about 8.4.

So if I were to follow the instructions for judging the leaching of the concrete tank I would never get it right, unless of course I collected a sample of that source water in a clean plastic or glass container and left it sit out too so I could check the pH of the source water again when I test the pH of the tank water for leaching.

I must agree though that the use of acid directly in an aquaponics system for adjusting pH once a system is up and running is not really a good practice. If you have no other choice but to use acid to adjust water pH for you aquaponics, please do it in a separate container so you can bubble the water and let the pH stabilize before you use that water for top ups. Better yet, try to collect rain water from a fish safe surface (galvanized, copper, or galvalume metals are not fish safe.) Rain water tends to be acidic and enough rain water can actually help balance the pH in a system that might alternately top up from a well. If your well water is liquid limestone and you don’t get enough rain, a reverse osmosis filter may be your only good long term choice.