Re: Does a tank of water weigh more if there are fish in it

Hi Pete,

Well, the simple answer to this question is... yes, assuming that we start
with a tank that contains a given amount of water - let's say 15 liters,
and we do not add or remove any water.  For simplicity's sake, we'll assume
it is pure water, and thus weighs 15 kilograms since one liter of pure
water weights 1 kg (note that fish do not live in pure water - they live in
water than contains minerals and such, so the water would actually be a bit
heavier).   Then, if we add fish and weigh the tank, it would weigh more to
account for the mass of the fish that were added.  The water level in the
tank would also have risen as the fish will have displaced some water (the
fish and the water can't occupy the same space at the same time).

Now, if when we added the fish to the tank, we also removed the amount of
water displaced by the fish so as to keep the water level in the tank at
the same level that we had before the fish were added, the mass would be
less than in the scenario above.  If the fish are neutrally buoyant
(meaning their mass is equal to the mass of water they are displacing),
then the tank would weigh the same as it did before the fish were added
since the mass of the water we remove would equate to the mass of the fish
added.

Here's an interesting aside: how does a fish maintain "neutral buoyancy"? 
Fish use an organ called the swim bladder to which they can add or remove
air, thus changing their buoyancy relative to their liquid environment. 
This is what enables fish to sleep in mid-water - they keep themselves in a
state of neutral buoyancy and neither sink nor float upwards.  If they need
to sink, they can release air and also use their swimming muscles to
swim down.  By releasing the air, it makes the work of swimming downward
less strenuous (kind
of like trying to hold a full balloon underwater versus a deflated
balloon).  If they want to float toward the surface, they add air to the
swim bladder and can also augment this with swimming.  

Thanks for your question!