Dave I couldn't agree more with you!
I also believe and from own experience there is no need to use RO water for
planted tanks (unless you are breeding apistos.discus etc.)
I have a 100 gallon planted tank which I've once mixed 50% R.O water but now
I use only tap water.
Our water is hard (GH 20d) but they do tend to get softened in the tank
after sometime (peat) I also have a CO2 system - do I get a PH value of
about 6.8.
Plants seem to be happy and growing well. The myth is what 'bothers' me
most. Is it better to mix like 50% or to use 100% tap water for the long run
?
I know this question is a bit wired and mostly depends on tap water
parameters. Is there any disadvantage using 100% tap water ?

"Dave Millman" wrote in message
...
Arnim wrote:

Now the real question: Am I doing my usual over-obsessing and worrying
about something that is probably fine?


Yes!


Okay, finally the water question. I was at first mixing my tap water
with
RO water 50/50 to get my KH and pH down so I wasn't having to inject so
much
CO2 to drive the pH from the usual high tap level of 8.2-8.4 down to 7
in
order to get enough CO2 dissolved in the water.

You express a common misperception here. No matter what your starting pH,
a
given amount of CO2 will drive it down the same amount. There is not need
to
"drive the pH from the usual high...in order to get enough CO2 dissolved
in the
water."

The best reference for CO2 issues is he

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm


This all seemed fine until
Carl at the LFS convinced me I should be using straight RO water
reconstituted with RO Right.


Things are getting worse, at least for your pocketbook. There is
absolutely no
justification for using RO water for aquatic plants. This is a myth that
seems
particularly prevalent at LFS, even good ones. Having said that, my
planted tank
is pure RO reconstituted with RO Right, but I do that for the sof****er
fish
(apistos) which I breed in that tank.

What is your goal? A healthy planted tank? Then measure your KH, and set
your
spiffy pH controller to give you 20-25ppm CO2. At your KH of 6, that means
6.8
or 6.9. Now you're done, sit back and enjoy the tank.


So what is my GH? Do I really need a TDS or conductivity meter? I also
read in the archived newsgroups that although very low TDS most
certainly
indicates soft water, high TDS does not necessarily mean high GH
(meaning I
could be measuring other non-GH type solids, sodium?). Does that mean a
conductivity meter is better. Would one of those $50 jobbies be okay
from
DFS?

You not need a TDS meter. Measure the GH out of the tap. If you dilute it
50%
with pure RO, GH will go down by 50%. With your hard water, you
undoubtedly have
plenty of calcium and magnesium for your plants, which is the only reason
to
think about GH at all.