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Re: (hardness, tds, etc.)

wshenefelt wrote:
> 
snip...
> I
> still have never heard if salt has the same osmotic effect.  I doubt it due
> to the molecular size but it sure is conductive.  Half a teasp per gal and
> your meter is off scale if it stops at 999 ppm.

The molecular (sic) size isn't a big factor in osmotic pressure, as much as
the ionization, hence conductivity. Ions is ions. The number is what counts.
Membrane *permeability* might vary a bit with ion type, but moving fish from
salty to non salty water can be disastrous for some species of killies.
Believe your tds meter.

In forward osmosis, as in RO, the primary movement through the membrane is
just the water, and not the ionizing salt components. Their charge is what
pushes the water around. If the ionic charges are matched on both sides of
the cell wall, there is little pressure created, and little water flow,
regardless of barrier strength. [The cell wall then needs almost no osmotic
pressure barrier. That's probably the normal condition for ocean fish, BTW.]

If the tank water has lots more salts than the blood, the water flow
initially is out and cells dehydrate, until the cell-wall osmotic barriers
can be formed. 

If the tank water has no ions (dissolved salts, tds), the water rushes in to
try to dilute the blood salts, and the cells may even explode if the
barriers are not formed quickly enough. Subtle damage is the norm, followed
by disease outbreak, gasping, etc.

The point of minimum stress usually is with *some* salt in the water. We
should not be surprised that it eases fish stressed by other factors.
Whether the dissolved salt is from sodium or calcium, chloride or iodide,
makes little difference in osmosis. It's the conductivity produced by
ionized salts that seems to relate most closely to osmotic pressure.

This is all a great oversimplification, for the gill and skin cells also are
trying to excrete ammonium and several other processes are involved. The
osmotic barrier itself is a complex system, involving at least three levels.

Bottom line is to keep pH in a region that avoids the toxic effects of
nitrites (low pH) and ammonia (high pH), and avoid sudden changes in the tds
of the water. The rest gets easy. ;-)

Wright

-- 
Wright Huntley, Fremont CA, USA, 510 494-8679  wright at killi dot net

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