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pH and soft water

My tap water has:

DH 3
KH 1.5
PH 7.4 or maybe 7.6

Is the only factor that can lead to this high a  pH a lack of disolved CO2?
 Is there something the
water company can add to the water to keep the pH high without changing KH
and DH?

I want to keep fish such as Tetras, Corys and Angelfish (and Ottos of
course) in my tank, but they
prefer water with a pH of 6.5 to 6.8.

My 48" x 12" x 18" tank has been running about  two weeks and now has 10
Cardinals and 4 Ottos.
 All are doing well.

DH 4.5
KH 2.5
PH 7.0
Ammonia 0.0
Nitrite 0.25 and falling
Nitrate more than 0 but less than 20
Temp 74 degrees

The substrate is a mix of 1/2" peat and 1/2" coarse sand mixed, covered
with 1/2" coarse sand and
about 1"  1/10" to 1/8th" gravel.

It is planted with about 70 plants including:
Vallisnera (corkscrew)
Amazon Swords 3 varieties different sizes
Sagitaria Subulata
Pygmy Chain Swords

and has 4 pieces of sand-blasted Mopani root.

The plants seem to be settling in well. One or two yellowing leaves, but
quite a few new leaves on the
pygmy chain swords. The hairgrass has grown almost 2 inches and is begining
 to emerge  from the water.

I have 120 watts of light and am injecting CO2 with a 'floramat CO2 manual
injection system' . I've also put
peat in my Fluval 304 cannister filter.

With all that peat and the CO2 I was worried I'd have pickled fish by now,
but the pH refuses to drop below 7.0

Using the tables on the Krib I see that I have about 7.5ppm  CO2 at pH 7.0
and KH 2.5
As far as I can see my tap water must have about 1ppm CO2 at pH 7.4 and KH

Does KH rise with the amount of disolved CO2, or does it remain constant?
Assuming it is constant and I
could get  to 15ppm CO2  my pH would be at 6.7.

Is that what I should be trying to do? I don't know how efficient the
Floramat system is, maybe I should go
for yeast  generation, but I was concerned about the variability causing pH
 fluctuation. I could use the
Floramat diffuser with yeast generated CO2, so I wouldnt feel I'd
completely wasted the 18 ($25) it cost :-)

If you are still reading ;-) I have another question... Can I use a lamp
dimmer to reduce the flow through
the Fluval 304? If I feed its ouput under the water surface I get strong
currents in the tank, which may not
suit Angelfish. To reduce the current I've created a long spray bar which
is directing the return water at
the back of the tank, just above the water surface. There is very little
surface agitation in this way, but I
guess the disolved CO2 that has made it's way through the filter may be
driven off as the water hits the
back of the tank.

Sorry for the long mail with lots of questions. I just want to be sure I'm
getting it right as I climb the very
steep learning curve. I hope I've provided all the information needed to
answer my questions. If not,
please fire away.


Mike Dodds
Fife, Scotland