Re: Tap water purifier

From: Sean Barry (
Date: Tue Dec 14 1999 - 16:38:45 PST

Date: Tue, 14 Dec 1999 16:38:45 -0800 (PST)
From: Sean Barry <>
Message-Id: <aabcdefg4139$foo@default>
Subject: Re: Tap water purifier

On Tue, 14 Dec 1999, William M. Gorum, Jr. wrote:

> them so well), and my fish and aquatic plants look really nice, but I
> don't know if I would use the water produced by one on my CP's. Yes,
> they are a cheap way to produce purified water without the waste of RO,
> and the water is wonderful for aquariums, but I am under the impression
> that the Tap water purifier uses an ion exchange resin. If that is the
> case, the impurities are pulled out of the water and are replaced with
> something else, usually some kind of salt. I've seen ammonia removing
> resins that are offered for use in freshwater tanks, and as they remove
> the ammonia, they replace it with NaCl (salt). Using salt water, even
> weak salt water, is not good for your plants.

That's true of most typical household water softeners, but you're really
not supposed to drink that water either so it's unlikely that tap water
purifiers (at the tap) would use that particular resin. Some resins
replace Ca++ and Mg++ and various negative ions (CO3--, Cl-) with H+ and
OH- (aka water), and it sounds as though the tap water systems may use
that kind of resin. Regular household water softeners almost invariably
use a cation exchanger that replaces only the positive ions, and then only
with sodium (Na+). Native chloride in the water is relatively innocuous
to humans and pipes when it is combined with the sodium ions, which is why
household softeners exchange using sodium rather than hydrogen (the
resulting water would be a solution of hydrochloric acid). The quick way
to tell if your treated water is OK is to use a conductivity meter, which
can be purchased quite cheaply from Fisher Scientific and other such
sources (standard disclaimers). If the water is softened by replacing the
native hard water with sodium, conductivity of the "soft water" outflow
will be about the same as the "hard water" inflow, just different ions.
Zero or almost zero conductivity means that (almost) zero ions of any kind
remain in the water. I wouldn't consider using any kind of home-treated
water for my plants (or my cardinal tetras, which spawn regularly) unless
I had a conductivity meter to check the product.

> Instead of purchasing an RO unit or bottled distilled water, try
> contacting your LFS about purchasing RO water. Almost all of the pet
> stores in my area sell RO water for $0.40 a gallon. All you have to do
> is provide the bottles. I get 5 gallon bottles from Wal-Mart and use
> them only for the plants.

These sources can be very good and inexpensive (RO water costs
$0.25/gallon at the supermarket where I live). The only problem arises if
you have lots of plants--lugging all that water home is pretty

I collect rainwater, buy RO water, and I use a home RO unit on water that
has been pre-softened in a household water softener. The RO membrane
under those circumstances lasts a really long time (several years so far).
I still run out of water from time to time....

Sean Barry

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