I've just increased the kH to 10 dgh in order to obtain 30 ppm CO2.

Why raise your KH to 10? Not that it's bad or anything but that won't
change your CO2 concentration. You need to add more CO2, nothing more.
What was your KH before? If it's anything over 3-4 you shouldn't have
any problems with PH swings while raising your CO2 levels.

But 20ppm shouldn't cause any BBA problems, not if it's stable.
Personally, I'd suggest correcting your phosphate deficiency first
before looking at CO2. If you haven't already done so, buy a couple
SAEs too, they will help a great deal.

hope that helps
Giancarlo Podio
www.gpodio.com