Just heard it from my LFS that my water is spot on. So that's a relief. In that I have done nothing major wrong and that I am alsao capable of doing water checks. There's hope for me yet. After much deliberating over corpses, fish videos, photos and listening to their entire medical and life history, we are now advised to try omnipur, since he reckons the underlying cause of all this could be parasitic. He reckons after 50% water change and carbon in filter last night, I am ready to go, so I am rolling my sleeves up and we're starting all over again. Better luck this time. He also gave me some of his antibiotic fish food, so ideally we have it all covered. Platies and catfish still happy as Larry, so that's good.
I shall keep you informed - keep your fingers crossed for me and the neons!