Yeah I know what photosynthesis is.
Co2 is a a big part of the photosynthesis possess.
You can make your own co2 with yeast if it's a small tank. I did the yeast DIY co2 in a 20gal long. Worked great got baby tears to carpet.
When you mess around with co2 you can run into lots of problems if you don't know what you are doing. It's a fine balancing act. You need to measure the co2 in the tank.
If you know about photosynthesis and how it works I will break it down easy.
Plants need light. The more light the more food they can produce and the faster they can grow. But plants use up more co2 when the light is stronger. Once the plants depleat the co2 in the tank they will not grow as fast or well even if the light is strong. So you add more co2 so the plants continue to grow fast and well. But all this fast growing takes up a lot of nutriants so if you have lots of light with lots of co2 the plants will depleat the water of nutriants quickly and they will not grow well. That's why you need to add more ferts to a high tech tank.
You have to have the right amount of light for the amount of co2 you are using and then you have to add just enough ferts to keep the plants happy with out feeding algea. Then you need to make sure the tank has good water flow but not too much surface movement. co2 is lost when the surface water moves too much. Water Surface movement gives you more o2 in the tank and let's co2 to escape gas exchage stuff.
Keeping plants is an art and once you master it you will find it to be easy it just takes some time to find what works for you.
The planted tank forum is a great place to start learning about keeping plants.
100Gal Cichlid tank, 80 Gal planted Community, 20Gal planted Betta, 7gal planted Betta, 20gal goldfish grow out. Yeah I like fish.