Very interesting, Tokis.
I, too, have been frustrated by the survival rate of some of the fancy guppies. One thing I have found is that they seem to do better in smaller tanks. I have bred fancy guppies, on and off, for many years, but have never had the opportunity to really "get into it" to the extent that I am now. And I am learning along the way. I always separate out the sexes as they mature, pick out the best individuals for myself, and take the rest to my LFS. LFS always appreciates males with nice finnage, females with color and large bodies. I was always able to get this on a small scale when I was working with ten gallon tanks. On a couple of occasions, though, I have recently had such large numbers of fry that I felt tens were cramping them, so as they began putting on size I moved them to a 55 to grow out. Both times I have done this, I have lost the majority of these fish for no apparent reason, usually shortly after the males began to get some size to their tail fins.
Now, this is just a guess on my part, and kind of a project I am working on myself, but it may be that the large flowing fins of the fancy guppies are to the point where it is too much for the fish to handle in a large open environment. As I said, I have successfully raised the previous generation of these fish in 10 gallon tanks...simple box filters, lots of floating plants, and religious partial water changes. 15 and 20 gallon tanks have worked also. But if you look at the depth difference between a 10, 15, or 20 gallon tank and the depth of a 55, plus the increased flow amount necessary for adequate filtration...then compare the finnage of a "wild-type" male guppy to the finnage of the fancy guppies of today...the fish just don't seem to be able to survive. My losses were much like the ones you described, there was nothing outwardly wrong with the fish, doing fine one day, dead the next. I think the answer may be in keeping these fish in smaller tanks, for whatever the reason may be.
