two qs: KH/GH, and CO2 valves

I'm moving in a week or so, and am preparing to set up another tank
when I move in. I plan to set up a welding-tank CO2 injection system
at that time as well. Naturally, the first thing I did was run out and
buy.. a bunch of books!

In the Tetra _Dwarf Cichlids_ book (ISBN etc available on request),
the authors list the water conditions in a number of fish collection
sites. Many - I won't say "most" - of these tables note a GH lower
than the KH. For example, 3.9dKH, 1.9dGH.

I understand that GH measures Ca++ and Mg++, while KH measures CO3--
and HCO3-, and that these values don't necessarily go in sync. As I
see it, this could happen two ways:

1) Some other cation is there instead of Ca or Mg. This could happen
with the presence of, for example, our friend sodium bicarbonate. If this
is the case, which ions? Na+? K+?

2) What Tetra says is KH is really alkalinity, and some factor other
than carbonate hardness raises the alkalinity in the collection
water. If this is the case, which other compounds (presumably
hydroxides of some metals or other) are present?

One other more mundane question that came up has to do with setting up
CO2 from one welding tank to more than one aquarium. Presume for a
moment that although I am not clueless in the physical sciences I know
next to nothing about valves or fittings. It is unlikely that I will
want equivalent amounts of CO2 injection in each aquarium.

My hypothesis for how I will need to set it up involves having only
one regulator for the welding tank, but one needle valve per fish
tank. So there would be a Y-joint 'after' the regulator, and each
needle valve (one per aquarium) would attach to the Y-joint. Is this
how it's done?  This doesn't seem to scale well, as needle valves
aren't cheap. What do people with N (N >> 2) aquaria do about this
problem? I'm interested in keeping the number of 50psi fittings to a

BTW, having gone through a few plant-related or at least
plant-interesting books lately, I will be posting reviews in a while.