| Fish do not have pristine water in the wild and as such their immune systems would be compromised greatly by being left in water devoid of almost all bacteria. It would be like being put in a medical bubble at birth and being let out at age 20... you would probably die from something as simple as a common cold or the flu. (The reason I used the analogy of being let out of the bubble is because there will, inevitably be some point inwhich that fish is exposed to something, whether it be from the air that contacts the water surface or something you introduce off of your skin, but it will eventually happen.) | 
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