CO2 in air

It takes very little CO2 in air to make a significant change in the
environments and growth of plants.  The first use of CO2 for growth
enhancement was in 40 foot by 120 foot or so greenhouses, and the co2
generator was a half-inch high flame from a little "alcohol lamp" containing
acetone, sitting in a pan of water.  The water was for fire safety. That is
not very much co2.  I have also known a person who was raising rabbits in
cages under their greenhouse benches, and the co2 from the rabbits was quite
effective in increasing the growth rate and general health of the plants.
  (No, they did not use the rabbit berries for fertilizer, and they did not
let the bunnies out to prune the plants.  :-D )

I would hazard an opinion, FWIIW, that just keeping a "lid" on the tank would
keep the co2 concentration of the air for the emersed plants significantly
higher than otherwise.  After all, much of the co2 you introduce into your
tank water is probably lost to the air. (partial pressure effects - basic and
irritating law of physics.) 

What the heck, the cost of putting a "lid" of glass over your tank is pretty
low, so I'd say "go for it."