The nature of baking soda is that it will give up H+ ions making more acids,
but only if the pH is above the buffer point. When it does this, it leaves
behind carbonate ions, which could precipitate as calcium carbonate,
(limestone), of calcium magnesium carbonate, (dolomite), if there is enough
carbonate and it is not being pushed back to bicarbonate.
You didn't say it, but I assume you are checking fish ponds with the same
test kit and not getting the same high readings, and that the pH test kit is
good.
You may be at some saturation point and major water changes could help bring
the pH down, though with your additions of acid and the temporary decrease,
I doubt it.
As for the maximum level, I saw somewhere, I think, a value of 300ppm.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html
"~ jan JJsPond.us" wrote in message
s.com...
Here I go again:

I've often heard Baking Soda will lower a high pH. I'm wondering just how
MUCH BS a pun? is needed? My lily pond is 3 yo 1,000 gallon EPDM with 6
fantail goldfish. I've added enough baking soda recently that the KH read
280/ppm today after adding 4 cups of BS. The pH is still reading over 9.0,
and I added 2 more cups this evening.

At what ppm is the KH detrimental to the goldfish, or is safe as long as
there is more liquid water than solid BS in the pond? At what ppm will the
pH dropped, or is this really an old fisherman's tale?

This pond has concrete block surrounding the top, but it is 3 yo, and I
haven't had this bad a time with pH the previous years, usually a few
treatment with M.acid has gotten it under control. This year I've used a
whole gallon on it (~2 cups/time). Only brings it down for a week or so.

The other odd thing, at least I find odd as I'm always preaching if your
water hyacinths are dying check the pH. Well the water hyacinths are
green,
not growing much, but green.... and so is the water, but that I don't find
unusual considering the pH. I'd also like to mention the duckweed is
growing great in the block holes, I guess a high pH doesn't bother them.
So
what do you think? ~ jan



(Do you know where your water quality is?)