Let me first note that with the popularity of the "Walstad Method" of aquarium keeping there is one mistake/misconception that often occurs, namely that the plants in a closed environment will sustain all of the proper chemistry with maybe a water change a few times per year. This misconception often leads to depletion of essential KH buffers and equally if not more important positive mineral ions.
This BTW does not mean one cannot maintain a vibrant planted tank as per this method, just that it is quite easy to outstrip natural bio available nutrients in a closed system, hence my comments.
Most photosynthesis occurs in the leaves of plants.) Chlorophyll has many different chemical make-ups, depending on the plant or bacterium that contains it.
This is an often misunderstood aspect of aquarium plant keeping.
There is so much anecdotal information that is out of date with more current aquarium/plant biochemistry information. Not only do plants need many of the minerals found in GH, but, just as important, potentially dangerous upward pH swings can occur if your GH is much below 50 ppm during peak plant photosynthesis.
For example, I observed a pH of 6.8 in the morning and then a pH of 7.4 in the afternoon in a tank where GH was low or almost non-existent.
You can sometimes tell if aquarium plants are photosynthesizing by observing the plants. When small bubbles form on the leaves of plants it is a sign that photosynthesis is occurring. This is commonly referred to as pearling.
This picture shows a foxtail with pearling under HO Planted Aquarium capable AAP SHO Lights