You can modify a powerhead rotor by either melting a lot of small holes in each paddle, or by slitting each paddle lengthwise and gently bending the two halves of each paddle apart to double the number of paddles. Those modifications make the powerhead chop the bubbles of CO2 even finer, so you almost get a "fog" of CO2 bubbles. This makes a "CO2 mist" as once advocated by Plantbrain. Those tiny bubbles circulate everywhere in the water, and even collect under leaves, where the plants get access to gaseous CO2 as well as dissolved CO2. The disadvantage of this is that the bubbles are visible, more so with high bubble rates, but visible anyway, and some of us found that less than appealing.