Disclaimer: I didn't know where else to post as this is a matter of interaction between light , fertilisers and CO2. I intentionally omitted the brand names because they can bias the readers. I will offer them after a time, if asked. I do not advise to expose aquariums in the long term to direct sunlight, especially in summer. Aside from heat, the amount of light varies greatly and cannot be easily controlled. I only use sunlight here to supply an "unrestrictive" amount of sunlight for the experiment. I will post pics when I get home, just to make the post better looking. Feel free to provide constructive criticism on the methods or conclusions. I am here to learn and to be exposed to others ideas. The setup: An aquarium with good water flow 10x, following EI dosing, stable good substrate brand X. Plants have established for more than 2 weeks. Receives good artificial light around 60 PAR, 6-8 hours per day from Led setup from brand X. CO2 is provided but unfortunately in an unknown quantity. Experiment: Sun directly shines onto part of the aquarium in the evening between 5-7 pm. Plants exposed are HC cuba, H pinnatifida, Hydrocortyle verticala, Althernathera and Weeping Moss ( so pretty wide range) Result: A few minutes after being exposed to sunlight, plants start pearling heavily (think 7-up look). Weeping Moss being a lazy grower only forms oxygen bubbles. A few minutes after the sun passes perling slows and stops, even though aquarium lights are still on. Notes: Without direct sunlight and in similar conditions slow perling is observed at the end of the light period, about 3-4h later. Discussion: The artificial light level is not high enough to reach the maximum rate of photosynthesis in given plants. When the sun shines, ample light is provided to the point light is no longer a limiting factor. Plants will rise the rate of photosynthesis until limited by one nutrient or biological capability is reached ( see the Weeping moss). As photosynthesis takes place faster the by-product, oxygen, will be generated faster than the rate of exchange at the water surface. The observable result of higher photosynthesis is faster formation of bubbles at leaf surfaces. Conclusion: As plants can go to a higher level of photosynthesis after increasing only light levels we can conclude that Light is the only limiting factor. To rephrase, there is a surplus of nutrients and CO2 already in the aquarium when the plants are exposed to direct sunlight.