Neon tetras dropping like flies

I bought 12 Neons for my new tank at the beginning of the month (just
over 3 weeks ago). The tank did a mini-cycle which I fixed by seeding it
(nitrites got to 0.1 so not massive). All settled down so I added 6
Panda Cories - 2 didn't make it but I got credit as the LFS believed
that there might have been a problem with the stock. I added 4 more and
these are all doing great. I also now have 3 Boesman rainbows in there
along with my two mystery fry....and 3 Amano shrimps...

The tank is approx 130 gall - 6 foot and has plants. Nitrite and Ammonia
are 0. pH is high(around 7.5) and the water here is hard.

Over the last week I have been losing the Neons - at a rate of approx
one a day. I am now down to 4 or 5 (it was 5 this morning but right now
I can only see 4). What I have noticed is that they seem to get white
blister like patches on their backs/sides. They then start swimming
erractically and then it is over within hours. No fluffy stuff or white
spots of any type. The majority I have found stuck to the inlet valve...

Just wondering if anyone has any idea what might be happening to these
tetras...all the other fish are doing just fine....the tetras themselves
were from the tank marked "larger Neon Tetras" so I'm guessing that they
are older than the more usual ones stocked but would be suprised if they
are dying of old age so soon after purchase and all within days/hours of
one another....

TIA
Gill