I've been on the well for 20+ years but my hubs was on the team that designed a new water treatment plant in the city and he says they strive to keep their outgoing water at 7.0. That may be different in other municipalities, however. Charleston's lower processed water pH could be due to the salt in the air getting into the tanks, etc. Just a guess.
Heck, I'M jealous of your 7.5-7.6!
I'm learning a lot about how water parameters affect fish, specifically Angelfish. I saw a page that indicated that Angels prefer blackwater tanks, which I'm not about to attempt, but aren't those tanks acidic due to the organic matter? Sometimes I think I should stop reading so much, it can be confusing.
Maybe it's the old thing about "grass is greener on the other side" but for years I wished I had water in the 6.5-7.0 range so I could keep SA cichilds. Over the years I have learned I don't really need that, I have been keeping Oscars, Angels, Convicts, Firemouths, Texas and Green Terrors no problems at all. Now I'm going toward African cichilds so I want my PH in the 8.0 range. :grin2: Can't make me happy I guess.
I also miss being on well water, the city does a okay job with water but we have two issues. Number 1 in the spring of the year the water PH goes real high because of the Chlorine they put in it (if you allow it to air off for 24 hours the PH drops back down) I think this is because of the extra rain and snow melting and number 2 once or twice a year you can smell the chlorine when you run the water and it gets a yellow color. They always say "it's safe to drink" and "nothing to worry about" but come on water isn't suppose to stink or be yellow. I just stop water changes for about 2 weeks or do very small ones. I'm very glad they don't use Chloramine to treat the water.
On the brackish tanks I have thought about doing one over the years but never done one. I agree the organic matter would cause the water to be more acidic and softer(don't even start on water hardness it makes my head hurt). That is part of the reason I don't do anything with salt water just too much to keep up with.
Just remember 99% of the fish we have in the hobby are farm raised so have adjusted to our water, and most of the books written are listing info based off the natural habitats that our fish never see. You will find large swings like you encountered are not the norm.