New Tank Syndrome
New tank syndrome is sadly very common to many people due to the lack of knowledge given, proper acclimation given to the fish, or if there is an extremely huge difference in water parameters that the fish cannot adjust to within the time given during acclimating. Sadly this is common not only with beginners but also with experienced aquarists who are in a rush or do not fully take the time to adjust a sensitive fish to their new habitat.
The common symptoms that a fish will show when added into the setup is that it will tend to either rest on the bottom of the setup, or it will stay at the water's surface. In many cases it will appear to be struggling to breathe (moving gills rapidly, sucking at air from the water's surface if possible) and will tend to hide away from any and all other inhabitants who might want to check out their new partner(s) added in.
Since this is not either a disease, an infection, or a form of bacteria but more of an organ issue when the fish is faced with a huge water parameter swing, there isn't much of a pathology besides what is happening to the organs when the fish is faced with different water parameters. The first thing that happen is the organs will force the fish to go into shock while it attempts to slowly adjust to the new water parameters (and this is where if the fish is weak immune system wise it will end up dead in this process).
By making sure that you follow all acclimating options that are available depending on your setup, you can reduce the chance or even fully avoid any new fish having new tank syndrome. The same can be applied when moving the fish tank to another location if it is fully established (as different cities can all have different water supplies which can vary greatly). If you want to view all of the options that are open to each step type, please take a look at either our Tropical Stocking Essentials or our Coldwater Stocking Essentials articles.