I find more often than not, when a fish dies, it has less to do with water parameters and more to do with it being unhealthy from the LFS
Some might not agree but I've seen fish big and small that can withstand a typhoon, some uncycled water wouldn't phase them in the least unless it was messed up bad and I mean bad. Fish are adaptive creatures, they are stronger than we think.
Buying fish at the LFS
is like a craps shoot and no matter how cautious you are, you never know what the health of the fish is. Now I automatically make the assumption that the LFS
fish has got something and proceed with lots of TLC. I suggest before you buy a fish to stare at it in the tank for at least 5 mins or more to really observe it's any unhealthy patterns. Look for hardy energy levels and activeness.