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Anonymous
What is the physiological difference that lets some fish live in fresh water, some in saltwater, and some both?

Could one 'acclimatize' a fish from one environment to live in the other by changing its tank's salt content slowly?
>> Anonymous
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Big big difference for most fish. Pure saltwater fish have huge kidneys and must drink water constantly, and filter out the salt and pee it out, because the ocean is saltier than the fishes body tissues. Pure Freshwater fish are pretty much the exact the opposite. However there are always exceptions to rules. Most notably Mollies and many "brackish" water fishes you will find in pet stores. Some Saltwater fishes can survive in a much lower salinity than the ocean, I have acclimated damsel fishes to as low as 1.012 density salinity. I have also taken the "freshwater figure 8 puffer fish" that you find at Petco, (they aren't actually freshwater fish, there is no such thing as a freshwater puffer), and acclimated them to a full saltwater tank, salinity was 1.026. This doesn't sound like a huge difference in salinity, but it is actually a huge difference, 1.000 is pure fresh clean distilled water, you will never see this because freshly distilled water will dissolve just about anything. Salt water, depending on the ocean, has on average 1.025 density, which is between 1/2 and 3/4 of a cup of salt in ONE gallon of water. So, short answer to the second question is yes, but it depends on the species, and there are not a lot of fish that will take well to it. It seems easier to acclimate brackish water fishes to saltwater, than anything else. Also, here's two of my fishes.