Can you explain this? What is causing this?

[Fshs]

So, this is an issue that I've had happen on three separate occasions with my community tank, and in all three incidences, the only fish affected were guppies. The incidents of this were spread weeks apart, so I'm unsure of the cause. Unfortunately, I don't have tank readings from those weeks to share.

The fish in the 15 G community tank at that point were:
1 black molly
1 pearl molly
1 black gold dust molly
1 balloon sailfin molly
1 swordtail
3 guppies (1 male, 2 female)
4 peppered Corydoras

Everything would be fine in the tank before I'd go to bed. I've finally managed to get all of the aggressors out of my community, so I doubt it's caused by an inhabitant of the tank. I keep a close eye on my fish, my guppies in particular, as they're the smallest in the tank. I feed them every night about 1 hour before the lights in the tank go out and ensure that everyone eats something. I'd wake up about 8 hours later, and there would be one dead guppy at the front of the tank. The appeared to be perfectly fine, no viewable problems on it's body, besides the fact that it's body was curved in a partial "C" shape (but not as pronounced, almost like a ")" instead of a "C"), almost as if it died just as it was moving it's tail to swim around an obstacle in the tank. It was always curved at the centre of it's body.

Since the guppies moved out of the community tank into a guppy-only tank, I haven't had this happen yet (but they've only been in the guppy-only tank for about a week).

Any idea what could cause this?

 

[Zagz]

It could be the guppies themselves are not great stock. Many guppies are cross bred so many times that their immune systems are weak. Are you getting them all from the same place?

 

[Fshs]

They all came from the same pet store, but at different times. The deaths were all over a week apart, and each from different shipments at the LFS. I go in on their "fish day" to get the best selection.

Is there any other causes that it could have been? Perhaps something I was doing wrong? I'm fairly new to the fish hobby (the tank has been running for 3 months).

All three guppies were different ages (apparent by their sizes and colouring), and it was two males and a female. All three had lived in the tank at least two weeks without an issue before the death, so I think that would rule out crap-stock, as they generally die in my tank within a week.

The issue was that no other fish were affected, and there was nothing wrong with the water. (If I recall correctly, all readings were good). Its been something that has been bothering me, because the only think all of the fish had in common was the position they were in when I found them dead. At the front right corner of the tank, curled in an ")" shape...