Did you add any/ enough salt to your water? I find that many lfs add lots of salt to their water for the guppies and when the guppies get into less salted tanks, they die and really fast - within one or two days. They get lethargic within hours but swim with a rocking motion and their fins disappear and then the fish just sink and die. All sorts of excuses from the lfs as to why they do that but the fact is, and Reg can attest to this, it isn't necessary to salt the water for guppies. I actually suspect that they do it so that once the guppies leave, they'll die and that's the lfs way of making you buy more guppies.