In terrestrial plants, when a certain temperature is reached, photosynthesis reduces/stops to help protect the plant. I recon you'd be boiling the tank before you see much change in aquatics. CO2 is used because there is light available at the chromoplasts that can be converted into sugars in the presence of CO2. Within a wide range of temperatures, the amount of light and CO2 available are going to be the biggest contributors to photosynthesis as far as I am concerned. Plants have longer term strategies for dealing with seasonal variances, notice how rose leaves go red when it is getting too cold, or how light coloured new leaves on a tree is, those are all strategies to deal with heat or cold.
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