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Re: NFC: aquarium basics site

"Brian T. Perkins" wrote:
> >
> >My RO (reverse osmosis) water, at 10ppm tds, initially is about pH=8. On
> >standing, that drops a lot, and with some peat, I can easily push it down
> to
> >pH=4. My 270ppm tap water, OTOH, is about 200ppm hardness (CaCO3
> equivalent)
> >so it is difficult to get it down even to neutral without creating a horrid
> >chemical soup.
> Thinking out loud:
> So, TDS is the sum of all the dissolved material in the water (metal
> salts,etc.) that would affect the conductivity of the water as read by the
> meter. 

True tds is *all* of them including silicates and others that contribute no
conductivity. [That's why I said it is only an approximation for "typical"

> The presence or lack of these materials would in turn affect both the
> water's buffering capacity (important to avoid wild pH swings and possible
> increase in ammonia toxicity, etc.) and the Hardness (presence or lack of Ca
> and Mg)

No. Only by coincidence. 

Buffering is a combination of an anion and its acid or some equivalent
thereof. This is often carbonate and bicarbonate. As more acid is added,
carbonate becomes bicarbonate by grabbing a hydrogen and the amount of free
H+ ions is not increased -- the pH comes right back up. Once the carbonate
all used up, the pH may "crash."

[That's only a big problem if nitrites are present. Just as high pH converts
harmless ammonium to deadly ammonia, low pH makes nitrites deadly. These two
phenomena plus tds (osmicity) shock give rise to nearly all the pH

Hardness is purely the divalent ions like Ca, Mg, and to a lesser extent
some like Fe. They are the things that defeat soap and make the water "hard"
to suds (the origin of "hardness," BTW)

Add all the NaCl (table salt o/e) to the water you want. The buffering will
not rise nor will the hardness, but the tds will go up dramatically. Soap
will still form suds. Adding a bit of salt to an aquarium makes the fish
fight less to maintain body fluids, so it is often a useful aid to their
immune system, if they get sick. It allows them to resist parasites like
"Ich" and "Velvet," too.

> So, if an organism has adjusted to a certain level of TDS in their
> water and then is subjected to a sudden change, then Osmotic shock
> (especially across sensitive gill membranes) can occur either killing them
> outright or weakening them to the point where disease organisms can gain a
> foothold.
> How am I doing so far? :o)

Fine. The osmicity is more related to the total dissolved stuff than to the
buffering or hardness compounds. Water across a permeable membrane tries to
leak through to dilute the side with the most salts dissolved, by developing
a pressure difference. [Reverse Osmosis is the same thing but extra pressure
on the saltier side makes it run backwards to purify water with dissolved

Body fluids tend to be more saline than fresh water, so fishes had to
develop an elaborate, three-level osmosis-control system to keep fresh water
from diluting their blood. This is a really neat trick since all breathing
and most waste removal must move through those same cells!

If you suddenly drop a fresh-water fish in sea water, the fluid rushes out
of the gill and skin cells and partially dehydrates them until the system
can restabilized for the new situation. It can cause some stress, but
usually not sudden death.

Going the other way, from salty or hard water to much *lower* tds, causes
water to rush into gill and skin cells so fast they rupture. That permanent
damage is often fatal. It also provides entry for bacteria and parasites
that otherwise would be rejected by the immune system.

The solution, when in doubt, is to drip acclimate for a couple of hours to
allow the fish to adjust the internal osmotic barrier system. This happens
slowly, so don't be in a big rush.

> Is there an easy conversion from microsiemens to ppm?

A simple factor of about 2 is pretty good. As I recall microSeimens is twice
the tds, but check me on that. [I didn't go look it up, and my memory is my
second shortest thing.] We don't need any great accuracy usually. Just don't
abruptly reduce the tds *or* microSeimens by a factor of two or more,
whatever numbers you are getting.

Don't forget. Many "puddle fish" and "stream fish" survive sudden rainstorms
very nicely, so small changes (< about 2x) are stimulating more than
harmful. Makes my Amazon Cories spawn, for example.


Wright Huntley, Fremont CA, USA, 510 494-8679  huntleyone at home dot com

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