I will respond to your question by explaining about aquatic plants a bit.
Plants grow by photosynthesis; aquatic plants need light and 17 nutrients to photosynthesize, and they will continue to do so until something is no longer available, at which point they will slow down or stop photosynthesis altogether until such time as everything is again available.
Light is the single most critical requirement; light must be adequate in terms of intensity (brightness to us) and duration. Different plants have different light requirements, so some will manage with less intense light than others. But the nutrients must be available with the light.
Some nutrients are more needed (quantity) than others. Carbon is one of these, and aquatic plants generally assimilate carbon as carbon dioxide (CO2). Some species can also use bicarbonates, but the majority use CO2 first, and a few (such as mosses) must have CO2. Other nutrients are also required; oxygen, hydrogen, nitrogen, and several minerals. These occur from various sources: water, fish food, plant fertilizers we add, organics from fish and plant waste, etc. Plants need these 17 nutrients in a fairly specific proportion. Too much of one nutrient will not help if something else is missing, and in fact it can cause even more trouble. Plants will "shut down" assimilation of some nutrients in the presence of too much of another.
Another danger is algae; it can take advantage when plants are unable to photosynthesize. To take one example, light. Plants need light in the red and blue wavelengths. Without these, they cannot use the light. Algae is less demanding, and can use any light. So if we were to use high blue light with little or no red, the plants would be struggling to use the light, and quickly give out. Algae would proliferate because it can use any light. Same thing happens if the tubes over the tank weaken to the point where the intensity is no longer sufficient for the plants; algae isn't fussy, and it will take advantage.
Balance is the key to all this. The light must be balanced by sufficient nutrients--all 17 of them--in order for the plants to photosynthesize. And some plants need more light (and thus more nutrients) than other plants.
I'm going to jump to the CO2 now, even though there is more I could say, but I think the above will be sufficient for you to understand what will follow.
CO2 occurs naturally from biological processes. Fish, plants and bacteria release CO2 constantly. When organics are broken down (decomposed), a lot of CO2 occurs. This is why we do not touch the substrate in planted tanks, we let the bacteria do its job and thus provide plant nutrients--not only CO2 but some mineral elements as well. With sufficient light and other nutrients available, plants will use pretty much all of the CO2. It is possible to have a natural balanced aquarium using mainly nature. You may or may not have to add some liquid fertilizer; you will have to provide adequate light. The majority of aquarium plants will be healthy and thriving in such a system. At first it may take a bit of experimenting with light duration and fertilizer to achieve the balance, but it is possible and not as difficult as this may sound.
As soon as you add CO2, you raise the level considerably; there must be more light, roughly twice if not more than what is needed without CO2 being added. And nutrients fertilization will be needed, perhaps even daily. It all comes back to that balance. If any thing is not sufficient, the plants can't continue.
I do not use CO2 and I do not recommend it, unless one specifically wants to propagate plants or have some of the more demanding species. To have a natural healthy balanced aquarium for fish it is not
necessary. It costs money to buy the equipment, and it costs more money to maintain the aquarium--double the light adds to your hydro costs, not to mention the CO2 and fertilizers. And there are issues with the fish from all this.
I'll leave it there for now.
You mention having problems with your plants, so from the above you will see that it can be due to light or nutrients. We can explore this, but first provide us with the light you have, what type, how many, how long is it on; and what if any fertilizers you are using. Also, the tank size, and fish and plant species. All of this is related.