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RE: Too much oxygen

> Karen Randall wrote:
> > I would wait 24 hours
> > if you fill the tank directly from the tap, particularly in 
> the winter.
> > The reason for this is that tap water, particularly in the 
> winter is high
> > in various dissolved gasses. (you see them collect as bubbles on the
> inside
> > of the tank glass as you fill the tank) These can be very 
> hard on the
> fish.
> Really?  A while ago, someone told me it would help remove
> chlorine/chloramine if I deliberately stirred up bubbles when 
> I do a water
> change.  So I've made a practice of it... does this create a 
> problem for the
> fish, or is it not a problem for just a 20% water change or so?
> Alysoun McLaughlin
> Wheaton, Maryland
Karen is referring to the supersaturation of gases in tap water due to
pumping under pressure and/or cold water heating up to room temperature.
Tap water distribution systems are maintained under pressure at all
times, both to insure adequate flow and to prevent polluted water from
outside the pipes to enter in at leaks.  Any additional gas introduced
into these pipes (e.g., a leaky manifold) will be dissolved at these
higher partial pressures, and will often be supersaturated when it
emerges from the tap.  Also, gases are more soluble in cold water than
warm, so when gas-saturated cold water emerges from the tap and warms up
in an aquarium, the water becomes supersaturated and can form bubbles.

The problem resulting from this phenomenon is called gas-bubble disease.
This is characterized by the formation of gas bubbles in the body
cavities of fish, such as behind the eyes (causing exophthalmia) or
between layers of skin tissue.  Small bubbles can form within the
vascular system, blocking the flow of blood and causing tissue death.
Worse, bubbles can form in the gill lamellae and block blood flow,
occasionally resulting in death by asphyxiation.  At 140% saturation and
higher, gas bubble disease can cause fish kills, although the effect can
cause some problems at 105-140% saturation.

Stirring up bubbles during a water change doesn't hurt your fish, but it
probably doesn't do much for them, either.  It may hasten the degassing
a bit, but you certainly aren't removing chlorine/chloramine by this
method.  I've seen this myth mentioned on the list before, but I never
responded until now, because it is beginning to be repeated.

Chlorine gas (Cl2) is added to water, where it immediately hydrolyzes
and ionizes to form hypochlorite (HOCl) and chlorite (OCl-).   If
ammonia is added, then the hypochlorite can react to form monochloramine
(NH2Cl) or dichloramine (NHCl2).  "Free residual chlorine" is defined as
the sum of hypochlorite and chlorite, which you may have seen in your
municipal water report.  Regardless of whether your system uses chlorine
or chloramine, these chemicals eventually break down to form chloride
and water (chlorine) or nitrogen gas, water, and HCl (chloramine).  They
do NOT "evaporate" when sprayed, as has been repeated on this list.
They react, typically with organics, to form simpler, more stable
compounds.  Chloramines are more stable than hypochlorite, so they
remain in the water a little longer.


Tchobanoglous, G. and E. Schroeder.  1985.  Water Quality.
Addison-Wesley Publishing Co.

Lamb, J.  1985.  Water Quality and its Control.  John Wiley and Sons.

U.S. Fish and Wildlife Service.  1995.  Introduction to Fish Health
Management.  USDOI Printing Office.