I would say your "dome" is too small to be giving you much diffusion into the water. This is also evidenced by the amount of bubbles of co2 that are visible. At the fairly low rate you are injecting, you shouldn't see any actual bubbles outside the bottle. Keep the cap but cut a new bottle to fit on it. You'll more than likely need about half the length of the bottle. The idea is to keep the bubbles in the bottle until they completely disappear into the water. Once they escape the bottle to rise to the surface they are basically wasted.