Re: Hardened, softened and confused...HELP !

> The tap water in my area has a carbonate hardness of 1.5dKH. Now, why
> should I increase it to at least 3.5dKH, apart from the danger of an
> acid drop?

I personally prefer 5 dKH since it allows me to achieve the pH and CO2
levels that I desire (pH 7.0, 15 pm CO2).  "Acid drop" is only a
problem where an aquarist does not change water very often and/or has
a high fish load.  As nitrification occurs, carbonate hardness is
removed.  After a certain point, buffering will be lost and the pH
will drop.
> Do plants assimilate better at hardness in the region of 3.5 to 4.0dKH?

Not to my knowledge.

> But I don't really see any real drop in the pH. With hardness of 1.5dKH,
> my pH is in the region of 6.7.
> In fact the CO2 table indicated that with hardness of 1.5dKH, the pH
> should be in the region of 6.6 to 6.8. And this will tie up to an
> optimal CO2 level at 7.0mg/l to 11.1 mg/l

I like CO2 to be a little higher (15-20 ppm) to get faster growth.
You may like it lower to reduce the amount of pruning you have to do.  

> Now, do you experts reckoned I should increase my hardness?

If you're happy with your pH and CO2 levels, NO!

> Please advise as this is quite confusing.

It's simple.  Decide on the pH and CO2 levels you desire for whatever
reasons.  Use a chart to determine what level of KH coincides with
those parameters.  Set your CO2 controller to the desired pH and the
CO2<->carbonate equilibirium system will give you the CO2 levels you