[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[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

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

Here is a really amazing answer from Roger Miller 
from another forum (Thanks again Roger!):


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 

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"
Aquatic-Plants mailing list
Aquatic-Plants at actwin_com