Well personally for me I like having two Qt for fish. By having two of everything. That includes heaters, power filter, powerheads, etc, I can transfer the fish to sterile Qt after two weeks of therapeutic levels of copper. The second tank is identical to the first. It’s just a sterile Cycled tank to transfer the fish to after the copper treatment. By having a second tank I can avoid keeping them in there the full 30 days of copper. I find some fish start to get bacterial infections from the copper if left in there for the full 30 days. This is just an opinion though. It really depends on what your aiming to do with the Qt process. And of course if you need to treat your fish with meds, you can’t really do that with inverts in there. So I’d say 3 Qt tanks. Lol!!!! One for corals and inverts. And two for fish. I have two 20 longs for this purpose. If it’s a new build, I usually quarantine all fish first. And then the clean up crew when needed. And the coral, when the tank matures enough. By the time I’ve quarantined all fish it’s usually several months down the road. Quarantining several batches takes time. I always like to add fish slowly over a period of months. That’s just my take on it though. You can certainly use only one tank. It just takes a little longer to push everything through.