Silly Question (there's no such thing)

Paul Schlosser SCHLOSSER at
Wed Apr 27 07:21:56 EST 1994

In <2pldrt$ijt at>
jprao at (Jay Rao) asks:

>I am not a student of Biology or Chemistry, so I had a question for
>these students.
>When we keep fishes in a Water Tank, why do we need air pump?  If
>we don't have air pump, why is there a need to change the water
>I thought water H2O contains O2, which can be extracted by the
>fishes.  Also, I thought Water is capable of absorbing O2 from air.
>If that is so, then there is no need to change water in the tank
>Another question is How do salt or clear water fishes manage to
>breath, if there is no recycling of water?
>These questions most probably silly.  But I do not know the
>If someone can answer these questions, it will be helpful.

Water does contain and can absorb O2, which the fish extract.
In a fish tank, we keep fish at a much higher density than usually
occurs in nature.  That is, if you take the total inches of all the
fish in the tank, and divide that by the volume of the tank, the result
would be much higher than in nature.  This high density of fish creates
a "demand" for O2 that is much greater than in nature.  O2 is only
sparingly soluble in water, and without an air-pump it tends to enter 
very slowly.  This normal rate is enough to support fish in nature,
where the density of fish is low.  Also, water falls or rapids in
streams and wave action have much the same effect as an air-pump: they
increase O2 absorption.

On a separate issue: the reason that you may need to change the water
in a tank regularly is *not* to replace the O2, but because it can
become fouled with the fish's waste.  (Note that the decomposition of 
these wastes can remove O2, adding to the demand.)  In a properly set-up 
tank, however, the water needs only to be changed once a year or so.  
The trick is to have a filter with activated charcoal that is replaced
every week or two.  The mechanical action of the filter removes large
particulates, and the charcoal absorbs some of the chemical wastes that
can build up.  There are also beneficial micro-organisms that will 
become established in the gravel or sand at the bottom of the tank 
(after a week or two) that will help to remove and de-toxify the wastes.
Books on aquarium keeping suggest that the tank be set up and allowed
to "age" for 1-2 weeks *before any fish are added* so that these colonies
can become established first.  The use of an "under-gravel" filter
improves the action of these colonies.  **The primary reason why a fish
tank may need to have it's water changed regularly, however, is usually
over-feeding, not the fish's waste.**  You should feed them twice a
day, no more, and the food that you add should be almost completely gone
in 10 minutes.  If you still see food floating around after 15 min, then
you are giving them too much.

In small fish-bowls, it is practically impossible to use a filter, and
so one must change the water periodically.

It is possible to keep fish in a tank without an air-pump.  You've
probably seen the Siamese Fighting Fish in small bowls at a pet store.
Other pet fishes can also be kept without an air-pump too, you just
can't put as many in the tank as you can with the pump.

More information about the Bioforum mailing list