It does apply to fish, but you mix salt into the water replacing the "stuff" in the water. Would you put a
SW fish into
FW? Of course not because the salinity content of the
SW fish is much higher than that of the
FW so that cells in his sensative tissues (gills, etc) will become damaged.
As far as not drinking
DI, it would be the exact opoosite of drinking
SW. Think about what happens when you drink too much
SW, you become dehydrated and die. Why does this happen, because the
SW in your stomache is at a much higher concentration than your body, so that you loose water as it leaves your system to dilute the
SW. The same goes for
FW cucumbers turning into
SW pickles
Lastly, I bet your
DI isn't as
DI as it could be. I am willing ot bet your filter doesn't remove everything. Also, true
DI is so pure that in most containers things will leech out into it raising the concentration of "stuff" making the water safer to drink.
Overal, drinking a glass of pure (abolutely nothing but H2O)
DI water probably won't kill you, but it is not nearly as good for you as a glass of wather with some things dissoved in it.