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How low can I keep my KH?

I know some, like Paul Sears, have said that one's KH can actually be kept
lower than the 3-4 generally advised without risk. I would like to know
just how low I can keep our KH given our setup:

-- We moved our 135-gal tank with the plants (not fish) to a city with soft
water one month ago. Old Plants and new tetras and corys doing very well.

-- We just got 8 discus and will be doing *daily* 20% water changes per our
breeder's suggestion  The water changes in a planted tank are not so much
aimed at removing waste to keep nitrates in check, but rather to prevent
the buildup of "stunting" hormones or enzymes that some fish secrete in
tanks that prevent them from reaching full adult size. That's the theory,
at least.

-- We keep (and used to keep) our (pressurized) CO2 around 10 ppm. Our
Barclaya gets holes if the CO2 is much higher; plant growth has been
exellent in the past with this level when we kept angels; discus may be
sensitive to high CO2; I've been using up some Flourish Excel, too.

-- 4 x 96W PC lighting (2.8 watts/gal). Good substrate & water-column

-- Tap GH is 41. Again, our discus breeder uses and suggests a growout GH
of at least 150. We are keeping the GH at about 150 ppm with magnesium
sulfate and calcium chloride.

-- Tap KH is 24 ppm (1.3 dH). I've been using sodium bicarb. to keep this
at about 30-40 ppm. pH has been about 6.9. Is there really a risk of a
drastic pH drop with a 20% daily water change? It would like to keep the pH
and CO2 low with this naturally low KH. Given daily water changes with tap
with this KH, how low can I safely keep my KH? Is 1.3 too low? With
supplementary CO2, does alkalinity (carbonate) serve any purpose other than
pH stability?


Jared Weinberger                    jared at brainyday_com
____________ http://www.brainyday.com/jared _______________