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low ORP in new tank

Hi all,

A question on ORP for the high-tech-types.  I've had a "high tech" (CO2, 
laterite) 60 gal plant tank running for about a year.  For the last 6 mo. I 
have had a Pinpoint ORP probe in, which consistently measures in the 
500-550 mV range, and begins to drop when the water needs changing. pH = 
6.9-7.0 in this tank.  I think these numbers are in line with what I've 
seen other people (e.g. George Booth) report.

Last weekend, I set up a new 75 gal tank (no plants or fish yet).  The 
setup is virtually identical to the 60 gal (CO2, pH control).  I moved the 
ORP probe from the 60 gal to the 75, and it is measuring approx 220 
mV.  The tank contains 58 gal of water.  Used 25 gal R.O. water, and the 
rest Los Angeles tap water w/ Amquel.  Substrate is 110 lb of epoxy-coated 
"aquarium gravel", w/ approx 75 g laterite mixed into the bottom 
1/3.  There are 3 large pieces of driftwood, and 2 rocks from the LFS 
(rocks may be basalt, but I'm not sure).  pH = 6.9 (controlled w/ CO2), kH 
= 4 dCH, GH = 5 dH.  Filters have been on for 4 days now, and the water 
looks clear.

My question is: does anyone know why the ORP is so low compared w/ the 
established 60 gal tank?  Is this usual for a new tank?  Is this telling me 
anything important?

Thanks in advance.

- Mark Gilmore
Los Angeles