Are you serious? If the pH of the tank is 8.0 and the water supply
has a pH of 7.4, each water change administer a pH shock.

I'm quite serious. A small water change isn't going to put the fish
into pH shock especially with just a .4 difference.

Also, 3 danios is not too few and it's very doubtful they were uneasy
about it. I have 3 danios too and they school and they are very
healthy. I think the problem was caused by something else, possibly a
compromised immune system from an existing illness.

It happens, sometimes fish just get sick, especially during the first
week or two at home. It's a stressful time. All you can do is check
your water parameters, the fish and your set up and see if you have any
flaws. DP did that and there wasn't really anything significant that
would likely cause problems.

DP, if you like the LFS you bought them at, you may not want to
discount them yet. One sick fish isn't all that uncommon. If you had a
few sick ones, that may be a different story. I've found it pretty
difficult to find a LFS that'll meet my standard of care and now that
I've found one I like, I'd probably let one mishap slide. (Though I
haven't had any yet, phew!)