PH Always increasing in recirculating system?

[Reverend]

yes it is a problem, I am assuming mind you assuming, that even thou the room is air conditioned temperatures still change with going in and out doors opening and closing etc, rain water tank is scrubbed because it is our drinking water, when it does rain you have to consider, the roof it falls on which is zinc aluminum then into a steel water tank. does this have any bearing on it, I don't know whether metals carry any acids extra, I read somewhere that if you test PH in cold water then test again after you warmed it the ph is different then re test it when cold it goes back to the original ph, if that makes sense. Isn't is amazing when we used to grow back in the 70's outdoors the plants were watered with the garden hose ph was never checked and by gum we had some ripper plants.