you need to be able to house anything that will go into your display. the size of your quarantine should make sense to be able to fill and dispose of the water needed. to be able to heat or cool it and to medicate it. i would not have a QT larger than my system. because the fish that will be housed in it may need to have medication that is added based on dose per gallon of water. more water more medication needed. you may have to do large water changes and that is more expensive for larger volumes. you want to be able to observe your animals and catch them . you typed 8 gallons was this correct?