What Went Wrong?

[DreaminginBlue]

Last week I finished cycling my ten gallon quarantine tank. It was running smoothly with a sponge filter and heater, and I was ready to put fish in it.
I started off by going to a big chain store (couldn't drive the two hours there and back to my favorite aquarium store that day) and buying one female gold twinbar platy. The store has awful conditions, and it makes me upset, but she seemed active and healthy, so I figured I could take her home and take good care of her to negate the effects of her time in that terrible place.
I acclimated her carefully, lights off, floated bag for 15 minutes, then added a half cup of tank water to the bag every three minutes, dumped half out, repeated, and then scooped her out into the tank without getting any bag water in.
She often hid, or hovered in one place, hung out by the surface, and didn't seem very hungry, but the water parameters were perfect and I attributed it to the stress of the move.
I came home today to find her lying on the bottom of the tank, dead.
I'm beyond upset, I can't figure out where I went wrong. I'm not sure if she was just sick to begin with, but I feel awful.
Please help me figure out what happened... I want to make sure if it was by some fault of mine I don't repeat the mistake.
Thank you in advance for your help.

 

[nirbhao]

It's possible that the parameters were just so different that they shocked her. Otherwise she was probably already sick.

 

[DreaminginBlue]

Thanks for the reply, Nirbhao... I'm guessing the latter was the case. I'll check with the store and see what their parameters are, just to rule that out, though.