Take it from me, I learned the hard way. IF you ever need to treat with copper and you have live rock or inverts...etc...the copper will kill the inverts and live rock....at least I've been told it will kill the live rock. You don't want to treat in your main tank if you don't have too. I treated for ICK in my main tank...and everything including the silicone (?) holding the glasss together gets stained blue and it's there forever. I have set up a qt as I learned the hard way. YES it is hard, really hard to catch a fish in a set up with losta stuff main tank, but it can be done if you are sneaky, and have two people....we have a 'method'...anyway, IMO I would use one, cost me big $$$ when I didn't and now afraid to add fish without one. Also use it for my hospital tank obviously! Lots don't, personal thing I guess.