Contingent upon who provides the advice, I suggest you weigh your options. Some will tell you never treat display tank, because it does this, or that, or this.
Truth is, many have never actually tried that, or this, and they are going by what they may have heard before or read. I have done both, treatment in display and out, and I will lay out the pros and cons of each. Again, this isn't stuff I read or heard, but have seen first hand in my house.
Pros of treating display-
Time. If you treat in the tank, you save a considerable amount of time. This is not so much of a benefit to your pleasure, but the safety of the fish. I will get to this more specifically in a bit.
Complete elimination. By treating the tank, you ensure every cyst that will eventually hatch, can not reproduce and die off. The other side of the coin is to let the tank run empty, or fallow, for 8 weeks or more. Even then, ich has survived the process, Meanwhile, where are your fish, and how "stressed" are they where you are keeping them.
Biological filtration. If you use cupramine, which is by far the safest product out there, it will NOT destroy your beneficial bacteria. Despite what some may tell you, I have done it myself, and to other tanks, with absolutely no spikes whatsoever.
Cons of treating display-
The following are what some will tell you are cons, which I will talk about, again from my own history.
Kills bacteria. Again, with cupramine, this is erroneous. The great saying comes to mind here, when the legend becomes fact, print the legend. I have done this numerous times myself, and never had any die off to cause a spike.
Ruining live rock. This is borderline true. If you treat the tank for a fish only, and a month later, decide to make it a reef tank, yes copper will leach and kill corals. However, I have used the live rock treated with copper at one point, and some time later, have used it in reef tanks, with no adverse affects. Again, time is the factor here, and I waited at least a year before using the rock in a coral tank. I also have a few buddies who have enacted the same principle, and have successfully kept reef tanks with it.
Pros of quarantine-
Careful observation. Assuming your fish are big enough, I am sure this isn't much a problem in your display.
Easier management of copper levels. Since there is no absorption, it is easier to dose, and manage. This is really the only pro I see.
Cons of quarantine or hospital tank-
Inevitable spikes of ammonia and nitrite, the two real killers. Unless you have your hospital tank running all the time, with some sort of ammonia supply, you will not have enough bacteria to support any type of bioload. Whats amazing to me is most of these people who tell you to quarantine, stress not getting fish after you first set up your tank for 8 weeks or more for stability and ammonia problems. Yet it suddenly becomes ok when you quarantine.
Less than average set up. Unless your quarantine has the same gadgets as does your display (skimmer, oversize filtration, sufficient hiding places, stable parameters) you are subjecting fish to the same very thing that people warn against with new tanks, or cost efficient set ups. If the name of the game is minimizing stress, then I believe these quarantine tanks, have stress written all over it. New home, and chances are, this quarantine tank is at best 1/3 the volume of your regular tank, so now you just threw size into the equation. Less flow means less oxygen, less filtration, and more cramped quarters.
This is my experience using both. I have lost more fish in quarantine treatment, then in display treatment. I used a 40b tank for quarantine, overpowered hang on filter, heaters etc, and have had fish simply transfer from display, and never eat again. Not to mention, in order to keep toxic ammonia levels down, I did 50 percent water changes daily (can you say one more source of stress).
So my practice is, quarantine new fish, one at a time, and move to display. If the tank becomes ill, even after quarantining, I treat the fish, in the tank. All of this is info is from my history and observation, not from what Ive read.
