Mine might drop off to 78 on a cold night,but its usually 79 during the day.
I've got a few snails from when I first got into the hobby,but I've also bought more just have them die within a week.
I think a lot of has to do with the snail itsself.Maybe there just old,maybe their sick.There not exactly like fish,Hard to tell when somethings wrong with a snail untill we pull the stinking corpse out of the tank.