All The Fish I Bought At One Time Died

[Xall]

I purchased 4 balloon mollies recently, put them in a quarantine tank. 2 days later 1 of them dies, then every day after that one more died until they were all dead. I did daily water tests with the API test kit, ammonia and nitrite zero, nitrate low. There's a few other mollies in there living just fine, and I acclimated the fish properly. Do fish stores just get a bad shipment sometimes? What could've happened?

 

[BerryAttack]

if you could give the stats of what your test that would be good
how long have you had the tank up
what type of cycling did you do 
how big is the tank
how did you add them into the water? float the bag then add water in or float the bag them dump them in... the best way IMO is to drip them in... you put them in a bucket with the water they came with, and drip water in with an air hoes, for about an hour and boom usually no problem unless your water isn't good.

 

[Donya]

Bad shipments can indeed happen. Fish can be stressed in shipping (temperature swings, waste buildup in the bags for too long, rough handling of the boxes, etc.) and the damage sometimes doesn't manifest itself immediately; it's always a risk when the animals have just arrived at the store vs. having sat in the store tank for a while. Online livestock orders shipped to your door have the same risk. Usually shipping problems show up within a few days or so after arrival. If the fish had been at the shop for a week or more then shipping health complications are a less likely candidate than disease or other sorts of environmental stress.
 
What was the nitrate reading roughly as a number? The color scales can be hard to distinguish well sometimes, but "low" is hard to interpret since it means different things to different people. Did the fish show any other symptoms physically or behaviorally, or did they just turn up dead with nothing obvious wrong?

 

[msebar]

Are the other mollies still doing well?

 

[Xall]

I don't remember the exact numbers, but by "low" I meant you can barely see that it's not zero on the color chart. So above zero, but less than 5.0 because it surely wasn't that color. Zero in the API kit for nitrate is yellow, it was yellow with a incredibly small tint of orange.
 
The fish showed no symptoms, I just woke up in the morning and found one dead each day. I looked at their fins to see if maybe they got attacked or something (although I don't have any aggressive fish in my tank) and they were all in perfect condition. They just died, it was really a first for me, typically when I get fish if they're going to die of something it's a day or two after. The last fish to die had fin rot, I could tell that for sure. But the others are a mystery.
 
The tank is 75 gallons, I  did a fishless cycle with fish food and tetra quick start. It's been cycled and inhabited for about 4 months. I have a rena XP3 as my filter, According to aquarium advisor my tank is 49% stocked.
 
All of my other mollies are fine still.
 
I usually do the floating bag and then add water, eventually dumping them in. I like the bucket idea, I'll be using that from now on. Seems easier and more safe than what I'm doing

 

[livelifelow]

Were all of the mollies eating?  Another thought, I've heard that mollies can be kept in brackish water, you might want to call the store you got them from and see whether they were being kept in brackish or fresh water.