+1 to above, adding 12 fish at once is a recipe for disaster. It will overwhelm your biofilter. 1-2 at a time is going to be your safest bet. Not to mention, if you quarantine 12 fish at once, and one has velvet, there is a good chance you will lose all 12.
I keep a 20 gal quarantine set up, cycled with LR and sand and a small CUC (and a couple peppermint shrimp, there were aiptaisia on the LR). Right now it's fallow because the last inhabitant had velvet

Observe for 6 weeks, no prophylactic treatment.
A drawback to this is that the fish have to be observed very closely and the aquarist has to know what to look for - which I don't, always. I lost my last two fish to velvet a week into quarantine. I think I could have caught it in in time to treat the first if I had recognized it better (I don't think I could have done anything about the second - poor guy went from healthy and eating to dead in about 6 hours).
The drawback to prophylactic treatment (as I see it) is that every drug has some negative effect and used improperly, can do more harm than good. Plenty of people kill fish with copper, it seems. I nursed a wrasse through flukes and a bacterial infection only to overdose it with an unnecessary follow up treatment of prazi-pro. It's up to the individual to weigh the pros and cons based on their own comfort level and lifestyle.