What size tank are we talking about? That will determine how effective a DIY system would be on your setup. On tanks of under around 30 gallons, it is easy to maintain high CO2
levels with a DIY system if you keep up with mixture changes.
What kind of carpet are you growing? UG? HC? How much CO2
you will need will depend on what plants you are trying to grow, and what kind of lighting you are using.
As for preventing mold, I haven't heard anyone actually have that problem in their tank, though I suppose it is possible.