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Re: High KH lowers pH?

* From: "Hopkins, Samuel" <Samuel_Hopkins at marconi.com>

 >Unless you have critters in your tank that really enjoy highly
 >alkaline conditions, don't think of testing it.  How are you getting
 >water that tests higher than 8.4?  Is that the pH of your tap water?

Apparently this is common in koi ponds. The koi don't mind the PH of 8.4. I
guess people super dose baking soda so that they get a high KH, high buffer,
and stable (8.4) PH.

My point is that if you have plants, such as Anacharis, that utilize the bicarbonate ion, the pH is not stable with a sodium bicarbonate solution. It will go up to 10 or more with strong lighting. Before I switched to using rain water, I have had to work with the local well water in Hinds and Madison Counties in central Mississippi, which has a KH of 11-13, due entirely to sodium bicarbonate. There is no measurable calcium or magnesium in this water. It comes out of the tap with a pH of 8.5 , and I have had fish die in it when they were in a moderately-lit tank with Anacharis where the pH got up to 9.6.

I guess this would also be ok for cichlids. The typical
pond plants are Hyacinth, water lettuce, and Anacharis. Some others are
things like lilys and cattails which I assume get their carbon from the air.
This stemmed from a message from someone who said that their PH was 9.5

I'm assuming that this pH was measured in a pond. I just don't think that adding sodium bicarbonate to this pond will help at all. By adding bicarbonate ion, it will temporarily lower the pH because the bicarbonate is in equilibrium with carbonic acid (H2CO3), and some of the excess hydroxide will react with the carbonic acid. but then the plants will utilize the bicarbonate ion, and crank out lots more hydroxide, and the pH will go right back up. I would recommend that peat or partially composted tree leaves be added to the pond to lower the pH. Floating plants, such as water lettuce and Hyacinth will also help by shading the water and reducing the photosynthesis and CO2 uptake of submersed plants. Since they get their CO2 from the air, floating plants should not contribute to raising the pH. In fact their roots may release some CO2, lowering the pH. The same would be true for amphibious plants such as cattails. Circulating the water with a pump would help. Many pond keepers circulate the water through a sort of "marsh" section where it runs over the roots of cattails and other emergent plants. The aeration of the circulated water will help lower pH, since water with a high pH will absorb CO2 from the atmosphere. Also, the roots of the marsh plants will also modify high pH.

and someone responded to use acid to bring the PH down and then add baking soda
to stabilize the PH at 8.4. I had thought that adding baking soda would
buffer against the acid and keep the PH at 9.5. I got a message from the
second poster and she said that adding baking soda will reduce the PH to 8.4
if it is above 8.4 and raise the PH to 8.4 if it is under 8.4. Baking soda
is used because it's highly available and very cheap.


Try an experiment. Get a cheap plastic swimming pool for little children , place it in a sunny location and put in some water and nutrients and some cheap "feeder" goldfish. Wait until you get green water and measure the pH. Then add sodium bicarbonate and see if you get a permanent lowering of the pH.

-- Paul Krombholz in muggy, but dry, central Mississippi