The goal of quarantine tanks is to prevent problems in the main tank due to the possible sickness of a new fish or aquaria brought home. A quarantine tank should be used before you introduce it to your main tank.  Aquarium fish are often susceptible to numerous diseases, due to the artificially limited and concentrated. Also, a new fish sometimes introduce diseases to aquaria, and these can be difficult to diagnose and treat. Most fish diseases are also aggravated when the fish is stressed from a new environment, other tank inhabitants or just travelling in the bag.