Never introduce a fish that may appear sick. This seems obvious, but I have seen too many times customers saying, "Oh, I just felt sorry for him." If you want to take a fish that you feel sorry for, put it in a quarantine tank. Or if Uncle Bob’s shutting his tank down and wants to give you his fish, and he hasn’t looked after his fish for the last four months, don’t necessarily accept them into your main display tank, put them in a quarantine tank first. You need to make sure the fish that go into your aquarium are healthy; this means they do what they are supposed to do. They swim in the way they are supposed to swim and have nice, clear eyes, clean fins, and scales. Placing a fish in a quarantine tank for 2-4 weeks will help to secure the health of the rest of your fish.

In the quarantine tank the fish can be easily medicated and problems can quickly be detected.

As per.com Paul Talbot
Taken from www.cichlidhub.com