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Re: ORP levels for Freshwater Planted Tanks

David Aiken wrote:

>>> Other comments made are that as the redox potential falls, ammonia
>>> becomes favoured over nitrate between +450 and +300 mV ( a later comment
>>> says at less than +350 mV), and Fe2+ over Fe3+ at between +300 and +200
>>> mV.
>>> Does +500 mV sound like a reasonable ball park figure to aim for? I hope
>>> so because I'd hate for you to stop reading this list <g>.
>> But plants cannot take the Fe3+, which means all liquid iron
>> supplement will oxide quickly.  You better have a good substrate that
>> can supplement the iron or reduce the Fe3+.
>I accept the point about Fe3+, but if you keep the redox potential low 
>enought to favour the Fe2+ you will also have it low enough to interfere 
>with nitrification. One could ask which is worse.


If I remember my chemistry right, then something is being overlooked here. 
I think you can tell even without a fancy ORP meter if the ORP level falls
to the point where ammonia is stable relative to nitrate (thats as high as
+450 mV in the material cited above) - most everything in the tank will be
dead because there's no oxygen in the water. 

Ammonia (or ammonium) isn't stable relative to nitrate as long as there is
free oxygen in the water.  The fact that Fe2+ is stable relative to Fe3+
at even lower ORP is somewhat academic. 

That doesn't mean that those conditions can't arise in a planted tank -
the substrate would be a likely place to find it.  It also doesn't mean that
an ORP meter won't read values lower than +450 mV in water with living
(probably stressed) plants and critters - disequilibrium allows for a lot
of unstable conditions.

So maybe a "right" answer is that ORP needs to be high (say +500 mV) in the
open water where oxygen is needed and it needs to be low (<<+450mV) in the
substrate where ammonia and Fe2+ are needed. 

Otherwise <shrug>, maybe it just doesn't much matter, because the meter's 
don't work very well and the numbers are hard to interpret.

Roger Miller

just a skeptic, I guess.