Moving house is stressful for anyone and especially so for aquatic animals. Water quality can be completely different from one aquarium to the next and often the journey from store to home can be problematic. Unless using a thermally efficient container such as a transport bag, fish can experience extreme fluctuations in temperature and light intensity that can also be problematic. Bag tariffs in some areas have led to the scenario of people transporting tropical animals in bare, clear bags in mid-winter against the advice of concerned staff.
To optimise health and growth rates, breeders raise their fish in frequently changed water with low nitrates. We do the same in our stores, together with keeping an eye on factors such as compatibility. All of this means that they can experience some shock when exposed to old water that is high in nitrates and phosphates that established residents have grown accustomed to. As with temperature fluctuations and bullying, this stress can depress the immune system and lead to disease. As with colds and flu, diseases such as "White Spot" tend to be present at low levels in most aquaria but are normally thwarted by settled fish.
Even the healthiest fish will suffer when exposed to unhealthy/unfamiliar water quality, temperature fluctuations, stressful transportation, aggression from established fish and overly hasty unpacking. The answer when new introductions fail to thrive is to rule out as many of these factors as possible and rather than blame the fish themselves, ask yourself what could be done differently to make the transition as smooth as possible. If there's a guarantee that can be given on all our livestock, it's that every animal we sell wants to live to a ripe old age. As fishkeepers it's our duty to provide conditions which allow this to happen from the moment that they leave the comfort of the breeder's facilities.