guppies died!

[ScuitoAmi150]

Wednesday night I found the new fancy guppie male that I bought last week dead in the tank with my teachers two guppies & four mosquito fish (I'm watching them over the summer). It looked like Dominik just died for no reason.

Shortly after removing him I noticed that my teacher's female and male guppie were acting weird. She was swimming nose down and he was lying at the bottom of the tank & had a hard time staying up right.

I put the two guppies into isolation & they both died that night. =[

All of the mosquito fish are happy and healthy swimming around like nothing is the bother.
The symptoms of the two that I saw while they were sick/alive seemed like they got Swim Bladder Disorder. But that isn't contagious so how come all of the guppies died & the mosquitos are still okay?

[Oldfishlady]

How big is the tank, how much and how often are the water changes and when was the last one, water temp, filtration and additive used.

Gambusia are tough fish and usually have a stronger immune response than pet shop guppies IME