Uh oh Tanked. I have to call it. Cathic 1, Tanked 0.
I found this online:
The answer to this question differs, depending on whether you're talking about saltwater fish or freshwater fish.
Freshwater fish do not actively drink water, but absorb the water through their skin and gills. On the other hand, saltwater fish do actively drink sea water. Their gills process the water and take out the salt.
The salmon is an interesting example of a fish that lives in both fresh and salt water, and consequently, they have the characteristics of both types of fish. Salmon are born in fresh water, and migrate to the ocean. While living in the ocean, they drink saltwater by opening their mouth; their gills then rid their bodies of the salt and minerals. When the salmon enters freshwater streams to spawn, they stop drinking the water and instead absorb it through osmosis (osmosis means that the water passes through the cells of the fish's skin into its body).
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Ocean fish have the opposite problem. Surrounded by salt water, their bodies contain a relatively lower concentration of salt than the ocean water. In this case, osmosis causes the fish to constantly lose water in order to equalize salt concentration inside and outside the fish.
To partially compensate for the water loss, ocean fish actually drink water through their mouths. To get rid of the excess salt they take in by drinking seawater, they excrete some salt through cells in their gills.
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Fresh water fish absorb water through their skin and gills, saltwater fish actually do drink water.
In saltwater fish, they have to drink because their body's concentration of salt is lower than the surrounding water. Therefore, they have to drink huge amounts of water every day to stay hydrated.
In freshwater fish, their salt concentration is higher than that of the surrounding water, and, as osmosis dictates, they absorb water through their highly permeable skin. To keep from bursting, freshwater fish actually have to excrete water, up to 10 times their body weight daily, unlike saltwater fish.