If you want un-killable schooling fish get Black Neon Tetras - they are unbelievably tough! I swear mine were from the cooling towers at Chernobyl
I'm not even trying to breed guppies - I stick exclusively to boys, but even so I've lost 7 out of 15 to date. The best guppies are the moggies but I used to have 8 really gorgeous fancy "veiltails" in red, orange and yellow and bright blue and yellow.
I lost the first bright blue and yellow guppy within days - he just seemed to waste away. I assumed he'd been injured during transport and blamed myself for being clumsy. A week later, I lost another blue from a mysterious tail-melting disease (not tailrot - this was faster and more incidious). A third blue developed a sort of "blister" on his tail. I tried treating him, but his tail also melted and he died a week later.
Over my holiday I lost 3 other guppies - orange and yellows, presumably from excessively high temperatures. A fourth was so sick by the time I got back, I put him in the hospital tank but he died the next day. Now I have an orange and yellow in the hospital tank with white sort of "fungus" on him (never seen anything like it before) and a yellow and white guppy in the main tank who keeps shimmying and darting for no apparent reason.
I had the "moggies" from the same LFS, but in another tank and half the price of these "pedigrees". If I lose all the pedigrees, I might just replace them with some more moggies since they seem a lot tougher.
Like you, I wonder what on earth I'm doing wrong, and more importantly, what I can do to stop them dying on me. Maybe there's some sort of genetic defect? I can remember at a hamster show hearing about breeders who'd lost a whole generation of hamsters due to so-called "lethal genes".