Onyx's effects on alkalinity

[Thomas Barr <tcbiii at earthlink_net>]

Background:
I have a 75 gallon tanks with 160lbs of onyx sand in it.
I added 4 handfuls of ground peat to the bottom layer and
about 2 liters of thick mulm detritus from another tank.

This has been the change in the tank's KH over 3 weeks:

Alkalinity: 48ppm-52ppm, Tap water at 52ppm(steady).
The peat might be counter acting the increases in alkalinity but it has not
moved significantly. The tap water is soft, around 3KH, GH is 5 that's
stayed the same as well.

I think if folks are worried about onyx's effects on KH, they may want to
add some peat. I have soft water and lots of sand and no effects on KH which
is the main cause for concern as it relates to CO2.

CO2 levels during the day when the CO2 reactor is on, 40ppm(no fish).
CO2 right before the lights come on, 14ppm. pH change is 0.4 units in a new
tank. As then tank matures, this difference will decrease.
That's not much in the way of CO2 losses since I'm not adding any for 12
hours.
The tank has a wet/dry, not sealed etc, too much splash over at the moment
which will be fixed today actually. So even in a bad case, the set up does
not move that much even with soft water and large flow wet/dry filtration.

I'm going to reduce the splash and re measure things.

A 0.4pH unit swing at night is not anything to worry about. But I'll measure
this again in a few more weeks and the gap will be less based on the other
tank's I've had.

Others have reported KH increases using onyx, the peat seems to balance the
effect nicely when added at 1 handful per 1 sqft  to the bottom layer during
set up. The long term effect on KH decline as does the peat effects.

This seems to work out well and balance things out nicely even in soft
water.

Regards, 
Tom Barr