Just because they are a moderator, it does not mean they know everything. And my post wasn't aimed at them directly. It was about anyone who was making comments about salt being used to treat fish, when those individuals have never used salt.
As for salinity changes in water. If fish are in freshwater and salt is added, they absorb a bit more salt and have a little more salt in their system for a few hours until their bodies stabilise. However, if you take a saltwater fish and put it in freshwater, the fish takes on a heap of fresh water rapidly (through osmosis) and its cells can rupture due to the sudden influx of freshwater.
So adding salt to a freshwater tank is less of an issue than adding freshwater to a salted or saltwater tank.
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And salt is a much safer alternative than any medication out there.
eg: malachite green & formaldehyde are often used to treat protozoan infections in fish. Both these chemicals cause cancer. Salt also treats protozoan infections in freshwater fishes and it does not cause cancer.