I don't know anything about fishes and aquaria, but I know that this is not true. Milk is nearly all H20.
my bad
Milk contains large amounts of proteins, sugars and fats, which are a valuable
food source for naturally occurring aquatic organisms including bacteria, fungi
and algae.
Usually the limited food supply in the water reduces or restricts the
population and growth of these naturally occurring organisms. A milk spill
however is an unlimited food source, and this results in a population explosion.
As the organisms break down the milk they use oxygen from the water. As the
population increases, the oxygen level decreases leaving none for the fish that are
present, causing them to suffocate.
tauranga.govt.nz