How to Lower KH without lowering the pH?

Question

I want to lower the KH because I want to lower the CO2 but I don't want to lower the pH.

KH -180ppm

pH -6.8

CO2- 48ppm

Isn't 30 ppm of of CO2 harmful to fish, bt my fish are fine

I found this formula CO2(in ppm)= 3*KH*10^(7-pH)

Answer 1

Hypercapnia (CO2 poisoning) should be far less of a concern (especially if your fish are acting normal) than the pH swings carbonic acid (dissolved CO2) can cause.  I would be very thankful that your KH is as high as it is, as it will prevent these large pH swings from occuring.  Until your fish start gasing at the surface, don't worry.