If one fish is infected with a bacterial ailment or parasite wouldn't all the fish in that tank be exposed? Removing only the one showing symptoms wouldn't help prevent the others from getting sick. I would think the entire tank would need treated and quarantine wouldn't be necessary.
On the other hand, if one fish is injured or suffers a non contagious ailment I could see quarantine to prevent other fish from bothering it.
I recently had a bout with BBA in one of my 10g tanks. I resigned to a complete tear down and sterilization with bleach. I removed the fish to a quarantine to let the digestive tract clear of spores while I cleaned the tank, gravel, filter, and decorations.
I vacuumed my 29g tank into a 10g to use as a quarantine. I used about 3g from the established tank and 3g of fresh water. Not much different than doing a 50% water change in the original tank. No, the beneficial bacteria didn't have a good colony, but they would be present in the water from the other tank to help with ammonia and nitrite.
Since I only had a mollie and two cory's I didn't run a filter, I did frequent water changes until I was ready to replace them in the cleaned tank. :fish-in-bowl:
Here they are back in the cleaned tank, looking good.
