I know of one or two exceptions to the claim that 24 hours of light is good for aquatic plants. Hygrophila polysperma and some other Hygrophilas on 24 hours light produce distorted leaves with no air filled spongy parenchyma between upper and lower epidermis. The leaf looks narrower, distorted and dark green because of the lack of any air in the leaf. Back in the days when H. polysperma was not on the Federal banned list I could always tell if a pet store kept their tank lights on for 24 hours by looking at the leaves on the Hygrophila. Also, the alga, Hydrodictyon, water net, can't form its nets with their six sided openings in 24 hours light apparently because mitosis is limited to the dark period. According to a paper I ran across, there are a number of algae, including Cladophora, that only do mitosis in the dark. I wonder if the absence of a dark period could kill Cladophora, if you kept it up long enough.