Any chance of a picture of the plants and tank?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
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Java Fern does not need supplemental carbon dioxide (CO2) and neither do any plants in an aquarium unless the tank is very heavily stocked with plants. There is ample CO2 in the water from the fish and filter bacteria, as well as in the air we breath and that gets into the tank water.
Too much CO2 can drop the pH unless there is lots of carbonate hardness (KH) to buffer the pH and stop it dropping. In my experience, Java Fern doesn't do that well in really acid water.
Check your KH and make sure the pH isn't dropping.
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Too much light does not harm plants. Heat from direct sunlight in summer can harm plants if they get too hot, but the water temperature has to go above 32C, and then all the plants and fish will suffer.