IMO, not so weird.My tap water (without ageing) is:
pH: over 8.4 (it went past what my API test could measure)
TDS: not sure yet; should hopefully know by tonight.
Here’s what’s weird, though: I aged some tap water and tested it, and the gH spiked to 10! How did that happen? kH went to 5. That confused me totally. The pH did drop to 7.6, though.
There have been so many weird variables and readings and that’s what made me think maybe RO or RO/DI would be the way to go.
Tap water is treated and can be loaded with CO2 (drop pH) or dosed with carbonates (raise pH). Back in the day, aging water was popular. After sitting for a few days, it comes into equilibrium with the atmosphere and settles at true pH.
If your water is testing at 7.6 and 5 KH after aging, those numbers make sense and is pretty darn good for tap water. But keep in mind it can change throughout the year. Seasonal changes, source water changes, treatment changes, etc.
I use RO, but my source is softened well water with very high KH (18). If I had your numbers and they stayed stable don't think I would need to.
I don't keep shrimp so am not much help there. Are you aerating that aged water?
And your tank pH/KH numbers are lower because the Fluval substrate is lowering the KH.