[mistergreen]

There are 2 issues.
#1. I think it's about the density of water that makes it harder for plants to absorb CO2 like that would in air so we in fact inject many folds more CO2 by volume into the water than there is in the air.

#2. CO2 concentrations are measure differently in air vs in water. In air it is ppm(v) by volume. In water it is ppm(w) by weight. So 300ppmv is only 1.3ppmw of CO2 in water.

ps. If you set up a low tech tank, CO2 is generated by the bacteria in your soil/substrate. They'll produce something like 20ppmw of CO2 which is way more than what's available from the air. You can search for my research in a DIY CO2 sensor to see the fun.