Our rule is, if we put a fish in a tank, be willing to loose everything in that tank. There are a few variations on that, like if the fish can be caught you can move it to a hosipital tank. In that event, we keep a airlift sponge filter in the sump of another tank so that it is always seeded and is ready to support a hospital tank. Hospital tank is easy to do. Sometimes a bucket, maybe a 20 tank that can be set up with tanks water. You then need a method to keep water quality (the sponge filter with an airlift) or several water changes a week with no feeding. Keep a close eye on ammonia, temp and PH and keep some Ammo Lock handy if ther is a spike. The fish will be calmer without light and can go without food for longer than you think. The problem with occasional hospital tanks is you often run a greater risk of killing the fish with stress and instability (temp, PH, etc) than the disease. The problem with keeping an established hospital tank is that it easily becomes just another display tank. We really like Marineland's Lifeguard product for a treatment.
Anther method of quarrantine is to somehow use your sump or refugium. You can keep a close eye on the new inhabitant, it is likely easy to catch and treat if necessary, and it offers a nice slow introduction to the new environment along with the advantage of having the stability and introduction to the new chemical environment. Beleive it or not, even LPS corals that defend their territory with chemical warfare can become used to other coral's presence with the slow introduction to the new "smell" of the new neighbors from a distant sump introduction.
As far as corals go, it is real simple to take some easy precautions from the spread of nasty stuff. Any new acropora wil ALWAYS get an interceptor dip before it goes into the tank. If you don't know what redbug is, you should definitely find out the easy way. All you need is a salt bucket in a stable temperature area (off the floor on the counter) with a very small powerhead. Acclimate the coral to tankwater about half full. Put about a 1/8" chunk of Interceptor in the water and let it there for 12-24 hours. Next, do a dip of your favorite solution and then inspecti it very carefully. Use a magnifying glass. You are ready to acclimate it to the tank. Our favorite dip is Two Little Fishies product named Revive. It covers a broad spectrum of problems, even though it smells like Pine-Sol. Lugols is another well accepted dip.
Rick and Terry