Why Do They Die In Pairs?

[Twoapennything]

I've noticed that my fish, when they die, seem to die in pairs. On Monday, both my cories died; I have no idea why. They had been fine up until that point and swimming and eating normally. I had a betta fish die a couple of months ago, and within a day the female betta had died as well. I've had several pairs of powder blue dwarf gouramis, and they have died in pairs, too. I had a small school of mollies die off together, while the rest of the tank was fine. Same with guppies.

Why is this? Is there a real reason for this, or is it just coincidental?

 

[Jason7894561230]

It could be beacuase of the water requirments are the same so if it lapses it takes both of them out not just 1 because they need both.
But it could also just be coincedental