Summary of correspondence regarding pH in reconstituted water.

Michael Lardelli mlardelli at
Thu Jul 16 10:20:41 EST 1998

Dear all,

Thanks for all your replies regarding the pH of fish system water
reconstituted from deionised water and sea salt.

I am sending out a collation of the replies that I have received - these
have been edited to preserve the anonymity of the respondents.


Original question:

I hope that you will be able to give me a little technical advice on
maintenance of pH for zebrafish culture.

To ensure water quality in our recirculating system we prepare our water by
addition of aquarium salts to deionised water. This is performed in a water
preparation tank by an automated system that gradually adds a concentrated
salt stock solution and monitors the resultant conductance. If this water
was simply deposited into the main system then the resultant pH of water in
the aquaria would be 5.5, so we correct this to 6.5 by addition of NaHCO3
in the concentrated salt stock (sodium bicarbonate).

The NaHCO3 addition is not a robust procedure - it seems that small
variations in the amount of NaHCO3 added can give considerable variation in
the pH. I wonder if anyone knows of a better buffering system to use. I
have seen that some aquarium shops sell a buffer system called "Bullseye
6.5" although the cost to us for the volumes that we need would be
prohibitive. The ingredients are not listed on the bottle so we cannot copy
it. I note also that Aquatic Eco-Systems Inc. sell "pH stabilizers" but I
have no experience of their use and I imagine that the cost would still be
too high.

Does anyone know of a better buffering system that we could use or how we
can correct our low pH? I would be very grateful for any advice!



What brand of aquarium salts are you using?  We use Instant Ocean sea salts
at 60 mg/L in deionized water without further buffering and our fish are
very healthy and breed very well.  Our recirculating system includes
membrane filtration, activated charcoal, and UV sterilization.


You probably will not like this answer BUT:


The main and perhaps only requirement for water for rerio is that it be wet.
The main reason for the popularity of this little fish as both a lab animal
and pet it that it very undemanding.

My water here in ..., is, as it comes from the tap
very hard and very alkaline (ph 8). I get a good grade of dog chow, put it
thru the food processor and have over a two year supply of fish food. 
My fish thrive breed and grow on this diet in the above mentioned tap 
water and a bit of neglect.

I am quite sure that more zebras are killed with tender, loving and 
dangerous care.

My advice:

Relax. You and the fish will both be happier.


You are right in that pH is "affected very sensitively" by sodium bicarb
(ie small amounts cause large swings in pH). I have read where catfish
farmers with recirculating systems use calcium hydroxide to increase pH.
The benefits of calcium hydroxide are that it helps remove carbon dioxide
from the water and also does not affect salinity as much.  I have "played
around" with the two and found that the sodium bicarb will bring the pH up
quickly and the calcium hydroxide "holds" the pH very well, with less
fluctuations.  You can try using a mixture of the two.

You can also use calcium carbonate (ie coral) to help stabilize the pH.
You can get some from Aquatic Ecosystems or Aquaculture Supply. I use 2.5 kg and
put in in a "bag" made out of window screen and place the bag in a tank
with waterflow and aeration (our system holds about 1500 L = 400 gal)
Since our pH drops drastically during the day (pH of our deionized water
is about 6 and we like our system to run at about 7)  I shoot down the bag
with a strong stream of water to agitate it and let the water turn milky,
twice daily. The coral is changed about every 3 weeks. You can also use
"dolomite" which, I believe, is just another form of CaCO3. I used to get
it from Marine Biotech who built our system, but I don't know where else
to get it.  They sell it at garden supply shops also, but this is in a
powder form. I prefer the "coral," though.

I also add sodium bicarb as needed, to the system. i kind of have a "feel"
for the amount of sodium bicarb to add, but we use a 1 molar solution and
add up to about 400 mls per day.


I would let the system run without the bicarbonate.  Our water is pH 6.5
before we add it to the tanks, and 5.9 while it is circulating.  Most
artificial sea salt mixtures are bicarbonate buffered, like sea water.  The
aeration of the water is the primary reason for the pH change.  More CO2 is
entering the aerated water causing the pH to drop.


I read your inquiry on the zebrafish discussion group concerning a method
for maintaining proper pH naturally and inexpensively.  Have you considered
the addition of a mesh bag filled with crushed coral to your system
reservoir?  Many people have had success with this method when the coral is
used sparingly.

[After further correspondence:]

Coral naturally "dissolves" a bit as necessary to maintain
a pH. Two words of advice, though.  Don't
go overboard on the amount of coral (it wants to bring the pH of the water
to 8.0 naturally, not 7ish) and rinse it REALLY well before adding it.  The
beautiful milky white cloud produced by unwashed coral, although not
excessively harmful, is a bit disconcerting for the animal caretaker as
their tanks turn white!


We also use deionized water to which we add instant ocean and bicarb.  I
don't remember the molarity of either off the top of my head, except
that it is 100 grams of instant ocean and 21 grams of bicarb per 100
gallons of water. pH comes out to between 6.5 and 6.8 every time, and
system pH stays that as well, as long as we are good about changing
water and keeping ammonia down.   I have heard of other buffering
systems that simply use a tank full of oyster shells, or limestone.

[After further correspondence:]

It seems that your ratio of bicarb to salt (18 to 500) is considerably less
than mine (21 to 120). When I started here, I went through all
the different formulations for fish water. Like you I wanted to use
deionized water with salts added. I found that people use a range of
concentrations from about 2.5mM to 20 mM
(the calculation assumes that ocean salts are 100% NaCl).  I
picked something like 7 mM, as I recall.  I then found the pH to be wildly
fluctuating, panicked and added a lot of bicarb; it still fluctuated
somewhat and so I doubled that (we use baking powder, no need to use
expensive chemicals). Empirically, our "fish water" comes out to about 780
uS, and our fish lay very very well.


That's all...................
Michael Lardelli
Department of Genetics
University of Adelaide
South Australia, 5005

Tel.    (Aust. = 61) (0)8 8303 3212  direct
        (Aust. = 61) (0)8 8303 5633  departmental secretary
Fax.    (Aust. = 61) (0)8 8303 4399
e-mail. mlardelli at

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