My fish keep dying

[Xx Emma x Lee xX]

I have a fish tank at my mom's and a fish tank at my dad's, both 10 gallons. My mom's is well established, but I've been struggling with my dad's ever since he tried to "re-open" it for business after a long repose since it was last running. We have a water softener here, and so we buy Poland springs water jugs. We tested water before hand (pH, nitrates, etc). However, the only fish that have lived since we stocked it is a dalmation molly, a female swordtail, and a rosy barb. My dad bought many of the fish as a surprise, without knowing their characteristics. I think I am going to bring the barb back (if I can do this...) as beautiful as he is because he needs more room to minimize any aggression. These three fish are the first fish we bought, however, and have lasted longer and despite many others (4+) dying. What could be the cause?  

[Frozen Mist]

It could be that the others are getting killed off. Molly's -can- be aggressive. (I had 3 at one point and the dalmation was a serial killer towards the other two)

10g also isn't much space. If you added too many too quickly the sharp rise in waste/chemical fluctuations could be what keeps killing the new ones off. Mollys produce sooo much, omg. -.-;;  

[Xx Emma x Lee xX]

The fish tank at my mom's is actually just a mom dalmatian molly with her many generations of fry. She stored the sperm and is continuing to spit out fry here and there. The dalmatian molly in my dad's tank is the father, as I separated him from the female. And they do create so much waste, I have a very happy pleco in with the fry family.

The fish that have passed have all been varieties of mollies and platies, for the most part; a silver molly, a red dalmatian sailfin, male swordtail, mickey mouse. This is heart breaking, especially because my dad meant for this to be a gift. gonk  

[Celestial Fireflies]

Frozen Mist

It could be that the others are getting killed off. Molly's -can- be aggressive. (I had 3 at one point and the dalmation was a serial killer towards the other two)

10g also isn't much space. If you added too many too quickly the sharp rise in waste/chemical fluctuations could be what keeps killing the new ones off. Mollys produce sooo much, omg. -.-;;

This. Bringing back the barb is a good choice, as they do best in a group of at least 5 or more, and something like a 30gal is more accommodating to that. In fact, I wouldn't be surprised to find that the single barb is getting nippy and killing the fish.

How many fish did you add at once, exactly? For a 10gal, it doesn't take much to overwhelm the tank. Have you done a water test since then?