Fish are not so delicate that a change in water temperature due to a water change will kill them. If you ran the tank at 80F and changed 90% of the water to 50F, yes it would kill them. But, being 10F off in temperature should make no difference to the fish, since the change is gradual due to the relatively slow rate of adding the new water. I find my fish love to swim in the colder water as it comes in when I use unheated tap water. So, those fish are getting zapped with a really sudden temp change.