In Europe as I know Fish importers often use as a prophylactic treatment "Preis Coly Konzentrat" http://www.preis-aquaristik.com/eng/hei ... ntrat.html
It consists of 100% plant concentrates (as Melafix or Garlic) - with antihelmintic effect and is not toxic too much (but neither trustworthy effective in all cases?!).
Most wild caught fish have a few parasites. Many wild caught fish build up tolerances to these parasites, but when introduced into an aquarium they can become lethal. If the fish does become sick, you will need to diagnose the illness and decide on a adequate treatment; it may be a good idea to feed a combination antiparasitic food ( e.g. a food mixture including both piperazine and metronidazole) and short (with Clout - has a reputation as a harsh medication ) or long duration baths (e.g. Flubendazole or Flubenol 5% etc.); in some cases you must gradually increased the temperature over 30°C. If fish come in with torn or frayed fins incurred in shipping use Melafix as per the manufacturers instructions.
The Big problem is Fish tuberculosis (Mycobacterium
spp.), but no so often with Wild caugt fishes, this is more the problem with farm raised fishes. I know some cases in even our small country with small number of aquarists, who fall ill (local cases of Fish tuberculosis) and have unbelievable, to 6 months lasting problems - they all get this illnes after paying no regard to warning them not to buy fishes in pet shops!
But we survive in most cases, our fishes do not.
Change in habit, producing change of function, is the main cause of the production of change in living structure. F. Wood Jones (1953) Trends of life