Re: Growing conditions for plants vs. algae

> From: Michael Irlbeck <u7211aa at sunmail_lrz-muenchen.de>
> I also have some floating plants.  Still unidentified. 
> They have nearly round leaves, about 3-5 cm in diameter and very long
> roots with fine hairs. The roots are very pretty and sometimes have a
> blueish shimmer to them when they break the light in a certain angle. 

How about Limnobium laevigatum ("Amazon Frogbit")?

> CO2 and KH: I once thought that I could save CO2 by lowering KH. 


> If you want a certain CO2 level in the tank, say 15 mg/l, you would
> have to add 15 mg/l plus whatever is needed to get past the buffering
> capacity (KH).

No, it's a continuous function, not a threshold. 

> So _I think_ it would take more CO2 to get to a certain level of free
> CO2 when the KH is high. 

Still true. 

> Maybe it's just the same ammount of CO2
> needed to reach 15 mg/l, no matter how high the KH is. Maybe the
> difference would just be in a different resulting pH.
True.  But you have a poor mental model of what's going on.  KH will
raise pH and CO2 will lower pH.  You can select any two of the three
variables to determine the third.  The KH/pH/CO2 relationship is fixed.

For example:

I like a pH of 6.8 to 7.2 for my fish and plants (personal preference).

I like a KH > 3 degrees so the CO2 test kit will read accurately. 

I like CO2 over 15 ppm for best plant growth and less than 30 ppm to
avoid stressing the fish.  

Since I have soft water, I can add baking soda to raise the KH to 4.5
and inject 17 ppm of CO2 to achieve a pH of 6.9.  

I you have natural KH in your water, you can add more CO2 or settle
for a higher pH or somehow remove KH.