A key to controlling diseases is preventing their spread. This is the essence of quarantine. Most people understand the importance of a hospital tank for isolating and treating sick fish. It is probably even more important to isolate newly purchased fish for a period of time. During this quarantine, the fish are observed for evidence of illness.
I have few enough fish that each batch has gotten its own tank. So I really have no experience with quarantining. Even so, I have gotten disease transmission between tanks (while taking precautions like using separate equipment!). I have heard experienced fanciers say they quarantine for a year and even read where one person said they never combine purchased fish with their general population. I have friends who have failed to observe proper quarantine procedures with devastating results: they lost nearly half their fish to a bacterial infection in a matter of days.
Well, a pragmatic approach is probably to plan on a 2 or 3 week quarantine. This is long enough to go through the life cycle of parasites like ich and for most bacterial diseases to manifest themselves. It won't eliminate the risk of introducing an illness into your fish population, but it may reduce it from 50% to 10%, which is well worth the effort.
In addition to isolating new fish, I have read numerous recommendations of treatment regimes that the fish can be put through while in quarantine. Unless you have an identified problem like ich or flukes, I wouldn't just indiscriminately treat the fish. The exception to this is salt; goldfish are pretty insensitive to salt, while some parasites are very sensitive. A veterinarian specializing in koi has recommended a tablespoon per gallon of uniodized salt for 2 weeks of the quarantine period. Build up to this concentration over 3 days (and decrease back to fresh over at least as long). Raising the temperature in the tank to 75 to 80 degrees Fahrenheit might also be good, as it will accelerate the parasite life cycle. Just make sure you have plenty of aeration in the tank and maintain good water quality. This period should be made as stress-free as possible.