Floating plants slowing dying - too low nitrate?
I have a small shrimp tank which has only a few plants like crypt parva, microsword, marimo, and some moss. Usually the nitrate reading will be about 5ppm or less. Recently, I added some floating plants like dwarf water lettuce, frogbit, and something else that I can't identify right now. But it looks like a mini version of frog bit with about 4 mm leaves, sort of like a floating clover with a tiny root system.
Anyway, it has been about a week, and these floating plants are not doing so well. For example, the edges of the dwarf water lettuce leaves are starting to sink underwater and turn yellow. Similarly on the "floating clover" plants, an entire leaf will turn yellow and spotty.
I read that floating plants are good at removing nitrates, and indeed the nitrate level is zero. Is this the cause, and should I start dosing fertilizer? (The underwater plants have some nutrients from root tabs) Or could there be some other reason, like wrong amount of lighting (the top surface of the water is only 1" away from my led light bar), slight amount of surface film (not enough to be obviously visible, but enough to prevent small bubbles from popping), etc.