Corydoras for harder water?

Hi
I've had a tank set up for about 8 months now (still a newbie ) and
lurking this group for 6.
Here in London UK the water is very hard and pH is around 8.2. I have
had several peppered corydoras but they have lasted only 4 to 6 weeks
before succumbing to what seems a bacterial infection that produces red
blotches. I think this could be the hard water and high pH not suiting
them and so stressing them out. Does this make sense?
Hence the question in the header. I really like corydoras and would like
to keep some, but if they are not happy with the local tap water then I
won't try any more.
Setup is 20 gallon community tank with 5 harlequins, 4 rummies, 5 neons
and a couple of guppy males and females. All quite happy and healthy.
One remaining peppered corydoras still healthy.
Thanks for any feedback.
Paul
--
Paul McGuinness