Wasserpest, your KH level shouldn't prevent CO2 from dissolving. I have 20dKH currently with CO2 around 20ppm (DIY) on a 29 gallon.
Mantisx, it seems very strange you only have 7 ppm CO2. Can you see the bubbles completely dissolve w/o escaping? Your bubble count isn't too low is it? You know, you might acutually have enough CO2. Using titration pH kits are not so great for calculating CO2 since they only go by 0.2. 0.2 can mean a difference of 10ppm CO2. Buying a different brand of KH and pH kits will help making sure you have accurate readings.