Early in my fishkeeping days, after I had killed many a fish due to ignorance about the nitrogen cycle, I was quite frustrated by my inability to keep new fish alive, particularly the more delicate fish like Angels and Rams, but also common fish like dwarf gouramis, mollies and swordtails.  Like most, I was buying my fish from the local Petsmart and Petco's, bringing them home, floating the bag, adding a bit of water, then letting them loose.  Within a few days, the fish would start developing sores or just outright die. 
 
I adopted an introduction method that involved drip acclimation and parasite treatment with salt in a quarentine tank.  I started to see better results, with not so many early deaths.  Then, I really got into German Rams.  I spent a small fortune on  them, with nearly all of them dying within a week.  I was quite frustrated with my apparent lack of fish keeping foo.  I happened to go to a small LFS that had GBRs.  They were almost twice as expensive as the others I had bought, and the return policy was even worse than the other place.  But, ever the one to waste money, I bought 5 of them.  Due to circumstances beyond my control, I couldn't do the drip acclimation, instead doing a pretty quick float-the-bag, add-some-water job, and released them into their new home.  And they took off, eating like monsters, playing, chasing, displaying colors.  They did not die.  I started to suspect that many of my problems were inherent with the fish I was buying.  So, I decided to buy an angel and a few gold rams this time.  Same result.  Healthy fish, without a big acclimation production. 
 
I have lost a LOT of fish over the years, and I now strongly suspect that the cause of most of the problems is not with my water quality or how I acclimate them, but with the source.  I have done a lot of reading, and found that many species of aquarium fish are now so inbred, such as the dwarf gourami, that they have very weak immune systems and are terribly susceptible to disease.  The fish are usually kept by the breeders and distributers in anti-biotic and anti-parasitic laden waters, to maximize the number of fish delivered alive to the end stores.  In the bigger chain stores, many of the fish that aren't sold in a given week die in the tanks and are continually replenished with new stock. 
 
Clearly, creating and maintaining a proper aquarium environment for fish is critical, but it does not guarantee success.