Thank you for your question! The effects of CO2 on a given plant sounds like an interesting research project.

Much of the CAUSES (College of Agriculture, Urban Sustainibility and Environmental Sciences), which the Master Gardener program is a part, research focuses on the intersections of urban planning & architecture, food systems, agriculture, nutrition, and environmental health in urban settings and not so much on marine/waterway sturdies.
However, the Smithsonian Environmental Research Center in Edgewater  https://serc.si.edu/gcrew primary focus is on aquatic environments, so could have some useful models.  For example, studies on whether higher levels CO2 favor invasive phragmites over natives by Patrick Megonigal https://serc.si.edu/staff/patrick-megonigal/megonigalp6122002 , Tom Mozdzer (Bryn Mawr) and other collaborators. 

As to setting up CO2 diffusion, often CO2 tanks with regulators, diffusers, timers and monitoring equipment are used in aquariums. There are small kits that supply a vessel with powders and tubing as a cheaper alternative to a pressurized system, but not sure how it would be regulated and levels measured. 
Consulting with aquarium specialists might be helpful in finding equipment that will suffice. There are some dedicated aquarium stores in DC that could help you connect with experts.
From an equipment standpoint, since brewers also use CO2 tanks and diffusers in their processes, perhaps the DC Brewers Guild https://www.dcbg.org/ could also be a useful source of small-scale practical methods & second-hand equipment.

Also, the DOEE Aquatic Research Education Center https://doee.dc.gov/arec has many tanks set up to local waterways conditions and might be a useful learning center.

Please keep us posted on how your testing comes along, and the methods you choose,

KatieMac, Extension volunteer team