It's not that strange. Fish are not all exactly the same. Some are tougher than others and respond differently to stress. Also consider the female may have been stressed from being with the male or that it may have taken longer to catch her at the LFS. Though I'm very sorry she died, it works out, strangely, for the best. it wouldn't be the best diea to get a pair as the female would constantly be chased by the male. If you want a mixed group of honeys, get 2 or more females for the male. That way his attention is devided amongst them and no individual girl suffers to the extent to which she would if she were the center of the male's attention constantly.