Adding New Fish to a Tank (a dangerous thing)

If you are a fish keeper you MUST at some point add fish to an aquarium. But how do you go about it without harming your already existing critters? Well, you MUST quarantine new fish for at least a week (longer if possible) and look for signs of disease. Most fish keepers KNOW this but few do it. I never did until after ONE addition. A few years ago when I had a ten gallon aquarium that was already stocked with neon tetras, cardinal tetras, serpae tetras, platys, guppies, and danios, I had an older cleanup fish die. I wanted to replace it with another because they are great fish to have and I like them. So I went to Petco and got a seemingly healthy fish from a seemingly healthy tank. I thought, "This fish is fine; I don't need to quarantine," so I didn't. Well, a few days later, I was feeding the fish and noticed one or two white spots on a couple of the tetras. At this point I KNEW I had made a mistake and started treating the WHOLE tank for ick. In the end I lost all six neon tetras, all three cardinal tetras, all the platys, all the guppies, all the danios, the new cleanup fish, and one of the serpae tetras (one is still alive to this day!). Since then, I have quarantined all new fish and intend to keep doing it. A few weeks ago I got some black skirt tetras that looked healthy, but they developed ick and died a few days later. If I had put them into my main tank, I would have not only lost them but also my danios. So, if you are buying fish, PLEASE quarantine!