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Well I always heard to have a night/lights-out for planted tanks, but of course that was for rooted plants in soil tanks. I get that a plant is alive and performing it's functions even in the dark, so I believed (and shared) the advise to have a dark time. If this supposition is not correct, what is the scientific evidence? Is down time just about the water CO2 level? And fish comfort?
What then is the ideal photo period for a rooted-plant NPT with fish inhabitants? Keeping in mind algae and any other important factors.
:confused:
Not to worry.

The scientific evidence that I have recently found shows that several aquatic plants will grow much better with a longer photoperiod (that is, 22 hr and/or continuous light). This information is very useful for those setting up refugiums (water purification systems using aquatic plants) and those that grow aquatic plants to sell.

For ordinary situations and NPTS, I would follow what CS-Gardener recommends. My own tanks run with a 14 hr day and a 4 hr siesta. Works great!
 

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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.
Now this is interesting! :eek: Thanks for the input.

There does seem to be an interesting bio-rhythm to algae. In addition to what you've written, the marine alga Gonyaulax polyedra bioluminesces in the dark, photosynthesizes in the day, and divides in the dark just before dawn. My textbook* goes on to say, "If Gonyaulax is kept in continuous light, these three functions continue to occur with the same rhythm for days and even weeks, long after a number of cell divisions have taken place."

As to aquatic plants, I would agree that some species may look different with continuous light, but that doesn't mean that they've decreased their productivity. If you can find a paper that compares growth rates/biomass production of aquatic plants that shows decreased growth with continuous light, I'd love to see it. So far, all I've found is either no change or a major (and statistically significant) increase.

*Raven PH, Evert RF, and Eichhorn SE. 1992. Biology of Plants (5th Ed.), Worth Publishers (NY), p. 578.
 
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