Hi,
Both the lack of oxygen and the accumulation of CO2 can be problematic. Balancing it out with the right amount of plants/algae is likely not easy, especially if you neglect the importance of the air compartment in the ecosystem. With reasonably-sized headspace air, you could create a buffer for oxygen/CO2, but it would still be walking on a tightrope. In the long term, you probably need to either balance CO2 production and consumption or implement regular air changes, similarly to the water changes in aquariums, to reset the system.
Both the lack of oxygen and the accumulation of CO2 can be problematic. Balancing it out with the right amount of plants/algae is likely not easy, especially if you neglect the importance of the air compartment in the ecosystem. With reasonably-sized headspace air, you could create a buffer for oxygen/CO2, but it would still be walking on a tightrope. In the long term, you probably need to either balance CO2 production and consumption or implement regular air changes, similarly to the water changes in aquariums, to reset the system.