Disease affecting livebearers?

[JuiceBox52]

So I brought home some guppies and ghost shrimp from my lfs and didn't quarantine them because all of my tanks were occupied. Three females and one male. Seven ghost shrimp. The stocking before them was
6 rummynose tetras, 4 neon tetras, 5 five month old mollies, 2 oto catfish, 7 adult guppies and one guppy fry.
Two days afterwards I notice that a two of the new guys didn't look so good, swimming slow, flat fin, and hiding a lot. Next day those ones are dead and the rest of the new ones and one of my old ones sick.... this continues until about 2.5 weeks later all of my adult guppies and two of my mollies have died. My tetras, catfish and shrimp are healthy as can be. What do you think ot could be?
Temp is 78-79f
Ph is 7.0
Ammonia 0.25 (it's a little high but I'm doing large frequent water changes and it wont go down)
Nitrate 30ppm
Nitrite 0ppm

 

[Essjay]

The first thing that occurs to me is that hard water fish died while soft water fish are fine.
How hard is your water? Your water provider's website may give your tap water hardness - you need a number and the units (there are about half a dozen units they could use). If they don't give it, you could phone or email them to ask, or take a sample of your tap water to an LFS and ask them to test for GH.

Guppies are notoriously weak fish these days; if you do have soft water, this together with the weakness of the species, could account for their fast demise. Mollies, not being as weak as guppies, survived longer.