I got them three days ago. The male and the one female who does not appear pregnant yet are both fine. I bought them from a store that is only a few miles from me and I know they use API Stress Coat like me so the water parameters should be close to similar. I have no idea if they use aquarium salt, I'll ask when I go back in tomorrow. That's a good idea, I would never have thought of that.
I wonder if it has to do with them being pregnant. The other guppy who died was also pregnant but not as advanced as momma guppy was. I wonder if it was a disease or if both guppies really were on the older side and age/stress of a new home got them. I suppose I'll never know. :/ I've been feeding all the guppies TetraColor crisps then switched to TetraColor granules so they wouldn't cloud the water as much. Plus a few bloodworms here and there. They never notice the food until it sinks to the bottom, though. I also tried feeding New Life Spectrum Small Fish Formula pellets but they just got sucked into the filter before the guppies noticed them.
The little fry is still alive though. Considering it's my first fry, I would not have expected it to outlive its mother.
I did order a book on guppies too, should arrive Mon-Tues. I'm hoping it'll have some good pics of pregnant guppies and the birthing process so I know what to look for.
Thanks again for your help, vette91.