Only reason I ask about additives is because there are some that can affect alkalinity, which is in direct relation to you pH. Liquid Amquel Plus for instance, if you were to stop using it you would likely see a pH increase as alkalinity is replaced by WCs. Who knows what other things are out there that I don't know about.
Basically KH = pH aside from many other factors involved. If you can monitor and log any KH changes than you'll better understand what's happening to your pH. If you see a rise in pH after a WC but no change in KH than you can conclude that it's another factor aside from an alkalinity increase. GH plays a role but not as substantial as KH. The CO2/O2 relationship also has an effect on pH. Vigorously aerate stagnant soft water in a 5gl bucket and watch the pH climb quickly. As well as the water treatment plant additives I mentioned, like soda ash, my tap comes out at pH8 and settles to 7.2 within 24 hours if left in an open bucket. Acids in my tanks settle that to 7, then CO2 settles it to 6.4-6.6. Alkalinity remains the same, every change in my case is due to CO2/O2/treatment plant additives/carbonic acids. In a tank that holds a certain pH level with it's carbonic acids, increased WCs can result in a slightly higher pH. A gradual increase in tank pH to the degree that you're seeing is usually indicative of an increase of alkalinity (KH). Pulling driftwood out or removing any other source of acids including any of your additives that might contribute can do it too.
Make sure you are seeing high KH out of the tap before you go peat shopping, that'll verify a reason for the action. test it yourself with a liquid KH reagent, don't have the LFS do it or answer whether or not they think it's hard or soft, ask here. One person's answer is questionable and they will likely try to sell you something to change it, something unnecessary or even harmful. It's not so complicated that we need anything an LFS sells to fix it, if it were even a problem.
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