hey snowman,
We had something similar, we purchased 6 guppies from animates and over a period of 2-3weeks they died and took out all but 6 of our guppies plus 3 neon, 3 rummy noses and a couple of small hara hara cats.
Not sure what it was, but some did the same as you have explained, some were fine one day and dead the next while others had the tail disintergrate, but left the sinewy bits that support the tail.
Conclusion, don't buy guppies from your lfs, and since this has happened we got a refund on the 6 we purchased and both animates here in wellington will no longer stock them.
no help, but at least you know you are not alone in it.
B