Hi ukefc. I had a problem with my neon tetras swimming with their noses pointing down for a while, and noticed that it was always after feeding flakes.
Somebody advised me that because I was sprinkling the flakes on the surface of the water, the tetras were taking in air when feeding.
I started to hold the flakes under the water for a couple of seconds, and letting them sink, and I've never had that problem again.
When the local shop tests your water, do they give you a copy of the actual water readings? If so, what are they?
I would strongly advise that you invest in a liquid test kit and take readings for yourself as you shouldn't be losing so many fish, and the likelihood is that your water readings aren't 'fine'.
Also, when you are doing water changes, do you bring the fresh water you are putting into the tank to the same temp as the water currently in the tank?
Hope this helps, and good luck. Please keep us posted.