hmm very confusing. well guppies can take a ph between 5.5-8.0 and platties 7.0-8.2 (
www.liveaquaria.com )
if you acclimated them right it shouldnt be a problem. i to have platties mysteriously die. i still do not know why. hmm. have you used any medications. ick outbreaks. anything? could you tell us how you acclimated them. is their a excess of a metal in your tank.
personally if it was platty you lost. i would bet my money on ph. my ph is high. all my other fish lived but i have lost 7 platties for a mysterious reason. just my opinion. i think its ph. has your water had any chemical added lately. water softener. how long have you had the platties. guppies? 5 years?
but you never know how old fish are when you buy them. were the fish that died purchased on the same day. possibly born on the same day. but like i sais my bet is on ph/old age. ill keep searching.