Why does after each water change one guppy die?

I have a medium-size aquarium and there are about 24 guppies in there (I got my guppies about 6 weeks ago). Every time I do a partial water change (I take about one quarter of the water out and put fresh water in) one guppy dies, usually within two days. In the last 5 weeks 5 guppies have died. I do water changes once a week. Whenever I do a water change, I mix Tetra Aqua Safe and Aqua Balance into the fresh water because I heard that this is necessary to make the water safe. I mix about half a spoonful of each in the fresh water. I wonder what I'm doing wrong.