The primary reason for a light for a fish tank is to better view and enhance the color of the fish and tank decor. The lighting is of little value to the fish, especially since fish have no eyelids and can really only rest in total darkness. Both fish and plants produce CO2. Plants require it, fish have no use for it. Only plants produce oxygen that both fish and plants require.
Fish can only consume oxygen.
Fish always degrade the water quality and the higher the fish load the faster it degrades the water. Healthy plants always increase water quality. In low-tech tanks, most filters do more harm than good. In a fish tank, the fish are dependent on a filter for water quality, in a low-tech planted aquarium the fish are dependent on the plants. As a general rule a newly set up fish aquarium can be the most difficult time because you have to deal with establishing bacteria to control the water quality.