Fish require sodium in the water to exchange for ammonia across the gills. Without sodium, freshwater fish cannot excrete nitrogenous waste (ammonia) and they will rapidly die. This is equivalent to sudden kidney failure in a terrestrial animal.
As SimonL mentioned DI and RO water also has little buffering capacity, but fish will likely die from the lack of sodium before a pH crash.
As SimonL mentioned DI and RO water also has little buffering capacity, but fish will likely die from the lack of sodium before a pH crash.
