The problem with adding it to the prep water, is that the pH will be 7+ and the carbonate won't dissolve at any useful rate. One way to work around that issue is to dissolve CO2 into the prep water to make it more acidic, which will help to dissolve the carbonates faster. A mixture of Sodium/Potassium Bicarbonate will be easier.
To be honest, I don't see the small drop in carbonate hardness being any issue. There'll be a small drop in CO2 that matches the drop in carbonate hardness, but if you're that worried, just do the water change with lights out and wait for the CO2 level to return to wanted value before turning the lights on.
If you're aiming for CO2 @ 30ppm with pH 6.5, you only need 2dKH (36ppm carbonates).
Last edited by Audionut; 07-10-2015 at 05:49 AM.
Reason: Forgot about bicarbonates.