High Ph Swings At Nighttime

Could really use some help with figuring out my water issues, please.

I have well water. For the longest time (years) it's been stable at 6.5 pH, 3 KH and 8 GH, no matter what time of day/night I do water testing.

Then I notice that some of my more sensitive shrimp start dying and I test my water from the tap and it's 6.4 pH, 2 KH and 8 GH during the day but is hiking to *at least* 7.6 pH (I haven't used my high range yet), 2 KH and 8 GH.

(One time, during the day, it oddly tested for 0 KH but that was only once.)

The same conditions are present in my tanks where I'm not using any ferts or buffers. I'm not using CO2. Some of these tanks have plants. Some of them don't. But the pH issue is the same regardless.

In some of these tanks are leaves and driftwood for tannins and still, it's the same issue across all these tanks.

I've been noticing the shrimp deaths at nights, so I'm correlating them to the pH elevations at night.

No other concerns are present, no ammonia, or anything else harmful in my tests. TDS is about 170.

I go through my API Master Test Kit tests and my GH and KH and what I mentioned are the only issues I found. I don't have any of the other tests, like calcium, phosphorus, etc. I don't know what it could be and what I should buy, if any.

My pH seems to be doing the opposite of what it's supposed to at night. It's supposed to drop, I thought.

And I'm aware that low KH means the pH isn't being buffered, but this seems rather extreme from a 1 degree difference.

Any thoughts? (I'm really not in the market to use CO2 right now. Not in the right place financially or mentally.)

I was hoping I could figure this out and address it for stability. I don't want to have to go through all the hassle and money of RO and remineralisation.

And if not, I was going to use peat but this is a lot of trial and error to find the right amount across all these tanks.

I'm also concerned that if I use enough peat to keep the pH low enough at night, that it'll cause it to drop severely low during the day.

Thank you.