I would suspect that the flow from the powerhead is not high enough. Most reactors of this sort are working on canister filters. When the powerhead is used it has very little actual flow compared to the canister. The tipoff, to me, is the large bubble collecting in the reactor. The reactor sized to fit the flow will be just about an even contest between the bubbles floating up and the flow forcing them down. What you need is a strong flow that swirls and breaks that large bubble up into small bits that are absorbed into the water and leave the reactor. Change in pearling may be caused by not getting the CO2 actually into the tank for plant use.
Powerheads move lots of water as powerheads but fail when used as pumps. Wrong design for that.