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[APD] RTFM and back to carbonate usage
>Your comment about
>others doing the research for you so you won't have to do
>it certainly could have seemed to some as, well, a bit
>provocative
Uh? I said I prefer asking questions over books, and
noone can ever convince me that is wrong, especially
not in the context of a mailing list. That would end
up with a mailing list where every question would
be answered with RTFM.
Aaaaanyway. Back to topic and some really nice
information.
Here is a really amazing answer from Roger Miller
from another forum (Thanks again Roger!):
"Daniel,
In my expeirence plant use of bicarbonate isn't very
significant in a low-light tank. Without enough light
to drive bicarbonate use, the plants are restricted to
using the CO2 they can get from respiration in the
tank and from the atmosphere. That process
doesn't change KH.
Even in a sunlit tank where my plants did use
bicarbonate out of the water there wasn't much
decline in KH. I think when plants use bicarbonate
the overall reaction when the light is on is usually:
2HCO3- -> CO2 (used by plant) + CO3-- + H2O
The amount of carbon in the water drops but the
carbonate on the right side provides the same amount
of KH as the bicarbonate on the left, so there is no
net decline in KH. The pH gets very high -- easily
over 9.
When the lights go out the reaction stops and CO2
from the atmosphere and from respiraton replenishes
the carbon supply in the water:
CO2 + CO3-- + H2O -> 2HCO-
This reaction allows the pH to drop back to 8.4 or
so, but still doesn't change the KH. Just how far
the pH does drop will vary depending on how much
CO2 the tank can get from the atmosphere and how
much respiration you have in the tank.
If you have very much calcium in your water then
there can be an overall drop in KH, but it isn't
directly from plant use. It happens when the
carbonate produced in the first reaction combines
with calcium and precipitates calcium carbonate
"marl".
CO3-- + Ca++ -> CaCO3
Usually the marl will not all redissolve after it
forms. If that happens then you will probably see
the marl and you will be able to measure the
effect on GH and KH. If you water has more KH
than GH then this process can drop the GH to
the level where the marl will no longer form.
Then the KH will stop dropping.
You can get a drop in KH from nitrification and
from some decomposition processes that produce
strong acids. The acids destroy the KH in your
water. This will happen with or without plants
in the tank and regardless of what your light
level is.
You can probably maintain KH in your tank
without water changes just by adding some
shells, limestone chips or marble chips to
the tank.
Roger Miller"
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