Truly a DAY OF DEATH for fishkeepers...

It seems that the last day has not only been bad for James.... while he lost
a betta, I've lost two neons in my tank (6 gal, cycled, good quality water,
was originally 6 neons and 2 platys).

Not a big deal, of course - but I'm wondering if it's my stinky pH, which is
now sitting at around 8 after I added a couple of white rocks (any idea what
they are?) to increase the kH, which was about zero. So, while my water's
buffering capacity will be good, I'm wondering why these neons have died...
kinda hoping that it's just survival of the fittest (I only bought them 2
days ago) and that they were probably ill in the first place, but was
wondering what people thought on here. Can a fish die purely because the pH
is a bit too high? I'm going to buy about 4 more neons so they can form a
school - the remaining 4 are very happy - but I don't want to bring them in
to a slow death by alkaline poisoning!

This tank's great, though - having gone from 2 months with a boring old
goldfish to 2 platys and a bunch of neons, it's making me keen to save up
and buy a huge tank!

thanks as always for your help, people!

R