It seems like most fish available in the marketplace right now are infected with multiple diseases. Velvet especially is running rampant. I know for a fact that 2 of the 3 closest fish stores to me have velvet issues. The 3rd runs low levels of copper in their system so if they have velvet or ich it may not show until you get them home and out of copper treated water. If you do need to treat the fish, most treatments are not reef safe so you would need to set up a QT anyway. You would also need to catch every fish in you DT which can be a pain and could require a much larger QT than if you are careful about what goes into your DT on the front end.
Aside from that, there are other benefits to using a QT. It gives the fish a chance to recover from shipping and adjust to the food you will be using without aggression issues from fish already established in the DT. With some pre planning, it can also make acclimation easy. I match my QT salinity with the salinity of the system my fish is coming from. This way I can just temperature float and put them into my QT without a long drip acclimation. I then gradually adjust my QT to match my DT so I can scoop them out and transfer them over with no acclimation.
What is needed for a QT? (Medicines, flow, hides, foods, etc.)
My fish QT is a tank, HOB filter, heater, lid, and a light. I only use the light when I am wanting a close look at the fish otherwise the ambient light is fine.
At a bare minimum I would get a Seachem ammonia alert badge, a copper product like Coppersafe, and Prazipro. The 2nd post in this link has a more complete list of what is good to have on hand.
http://reef2reef.com/threads/medications-to-keep-on-hand.213574/
For hiding spots, I use cheap PVC fittings of various sizes that I picked up at Lowes.
Are some things ok to go to the DT immediately?
I don't think I will ever put anything directly into my DT again. I have added inverts to my DT directly but with ich and velvet so prominent right now, even that is a high risk unless the inverts come from a fishless system.
Corals can come with plenty of pests. Not only can they bring in Ich and Velvet, but they can bring in lots of other pests, too. Putting them in a QT gives a chance for any eggs or encysted critters they may be carrying to hatch since these won't be impacted by dips.
What size should your QT be?
I like my 10g QT. Big enough to handle most immature fish but still easy to set up and take down. The smaller water volume also means I need less meds when I treat.
What's the most livestock to keep in a QT?
It really depends on the size of the QT and the type of livestock.
