Lost entire generation of black mollies

[meadeam]

I had around 15 black mollies in a a 36g.  They were all from the same generation, and fully grown.  A few of them reproduced again, and 4 of those offspring survived past the eaten-by-parents stage.  Then the first generation started dying off one at a time.  I'd notice a fish looking lethargic, or sitting on the bottom, then in a day or two it would be gone.  Naturally I checked water parameters; no ammonia or nitrite, small amount of nitrate within acceptable range, no chlorine.  75-77f.  My water is hard with low buffer (I will look at the exact numbers).   When fish kept dying, I did a water change regardless  and two rounds of API General cure followed by the requisite water changes.  Fish kept dying until there was only on left.  I thought he made it, but 10 days after the second-last of his generation he also died.  They were around 1 year old.

The 4 offspring of that ill-fated generation are still in the tank, and have never shown any signs of illness through it all.  They are less than half-grown at this point.  Flourite substrate, moderately planted, Oase filtosmart thermo 100.  Other inhabitants include a few stray shrimp, and about a dozen mystery snails that were quite small when the fish were still alive and are reaching full size now.  I moved the snails in because the tank is rimless and I wanted them in a place I could easily find the eggs before they hatched.  

Any idea what may have happened?  I have a different 29g tank with tetra, rasbora, and corydora with some approaching 4 years old.  Maybe mollies just don't like my water?  

[Chick-In-Of-TheSea]

How is the pH?

Did the filter have any downtime/power outage?

Could it be possible to have missed the dechlorination step during water change?

Summoning Dr. @Colu!

[Colu]

A little more information would be helpful what your kH and GH and pH were they still eating ok before they died any white stringy poop rapid breathing hanging near sunken belly