This analogy does not hold up as it relates to ich. Cancer is not contageous. If one of your fish in the display gets ich, than all of your fish in the display will likely become infected. If you get cancer, you are not going to give it to someone else. Also, quarantine sometimes does not work in discovering a low level ich infestation, particularlly when, as often with ich, the parasite remains relatively confined to the gill area where you cannot see the parasite. Studies have shown that approximatley 1/3 of all fish sold retail have a parasite. Many times such parasites are missed in quarantine. Inadvertantly, hobbyiest then introduce these parasites to their displays even when quarantining. The question you have ask whether to treat regardless of observance of a parasite is whether you want to risk your other fish in the display and how onerous it will be to treat all of your fish in the display if the display becomes infected with a parasite. Longer quarantine times will minimize the chances of missing a parasite, but even then longer quarantine times sometimes are ineffective at discovering a parasite. You treat new fish regardless of whether they show signs of a parasite not only to rid your new fish of any parasites, but you so treat to also prevent your other fish in your display from being infected with parasites from the introduction to the display of the new fish. Once you have a display full of fish which has been infected with a parasite, it can often result in fish losses, diminished health of the fish in the display, and quite an onerous task to rid your display of the parasite. As the old saying goes, never bet what you cannot afford to loose.
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