Some people will treat all new fish with an anti-parasitic medication even when no symptoms are present, much in the same way that cats and dogs are routinely dewormed. Wild-caught fish are very likely to have parasites but show no symptoms, and may infect the less-robust domestic-bred fish which can quickly be overcome. Or the wild-caught fish may not have been exposed to pathogens the domestic fish carry and may be overcome. It is especially important to quarantine new fish when they are intended for a tank already containing valuable fish, which would be difficult to replace. Live plants may also be quarantined to help prevent the infestation of snails and other harmful organisms

i started using this way of quarantining fish just a couple of drops of methylene blue in the bag of new fish left to stand 1/2 a hour flouting in the main tank don't no if it works but i don't lose fish this way and it sterilizes the water before adding to the main aquarium. good look. remember just a couple of drops i take no responsibility from fish deaths
also if the tank don't look healthy i will do the entire tank to be on the safe side because i got no room for a quarantine tank