I'm afraid I'll kill my plants with a lot of light
Welcome to the hobby and the forum. Don’t be afraid, the hobby should be enjoyable. The setup in the photo, with a typical LED desk lamp over a small tank will typically put it in the low to mid light values... lots of variables though...very far from the light saturation point. Hygrophila polysperma(?) should do ok with the low light you have. There are lots of others aspects that help plants to grow, have a look at the article section
Articles / Tutorials . No need to get everything perfect the first time, some plants are hardier than others.
The practical points are above, feel free to disregard the finer details below... The light saturation point is just the light intensity above which extra light would be wasted as far as the plant is concerned. The plant tissue can’t use the extra energy, either due to metabolism (shade plants) or due to temperature or CO2. Contrary to what some people repeat (probably an obsession), plants like the extra CO2, the rate of photosynthesis increases with higher CO2 up to some saturation concentration. And that would explain while the study reports so much lower light saturation points for submersed growth, it’s highly likely that the experiments were not run under CO2 supplementation.
Exceeding the light saturation point, which is common in sunny locations, does not mean the plant will die. In some cases, if the light is too much and the plant can’t use it, it will start to produce pigmentation to protect itself, but even so that will only be the case for the top leaves. Lower in the canopy the plant will still make full use of the light (self-shading)
Worth mentioning, that this hobby is all about specifics. Hygrophila and Limnophila are diverse genera. Take for example the light demands of something like Hygrophila polysperma vs. Hygrophila lancea ’Chai’. The paper’s author gives values at the genus levels which doesn’t really inspire confidence.
