julielynn47 said:
The minimum hours I have heard of is six. George Farmer wrote this in an issue of PFK, in response to a question involving algae problems I believe.
A quick explanation may help. Light drives photosynthesis in all plants, which includes algae, and in photosynthetic organisms (like cyanobacteria). Aquatic plants will photosynthesize "full out" provided the light is of sufficient intensity and all required nutrients are available. However, there is a limit, because plants, like animals (including our fish) must have periods of rest every 24 hours. In the tropics, daylight and darkness are constant throughout the year, with roughly 8-10 hours of each, and the remainder the dawn/dusk periods. However, daylight does not always mean full sunlight, but rather light of sufficient intensity to drive photosynthesis in the particular plant species. Some plants grow well in shade, while others need more direct sun; aquatic plants are the same as terrestrial in this respect. Light and nutrient needs vary from species to species.
Photosynthesis slows once some requirement is no longer adequate--what we term the Law of Minimum. This is when algae has the advantage because algae is not anywhere as fussy over specific light or some nutrients and can make do with deficiencies that higher plants cannot. This is the balance about which I write so often.
So, whatever amount of "daylight" you provide, being tank lighting (like sunlight in nature), and if all the necessary nutrients are present, plants will photosynthesize (grow). The trick is to find the light duration that balances the available nutrients so plants use these but algae does not. Adding nutrients is relatively simple, except for one, carbon. We rely on natural CO2 (except when using diffused CO2 which takes us into a very different and much higher level of balance) and the trick is to know when this is running out, and stop the "daylight" accordingly. This is usually why extending the "daylight" can cause algae; nutrients, especially carbon, have likely run out. Remember I said at the beginning that plants will photosynthesize to the max so long as everything is available, so they do not "go slow" normally, and CO2 being a very important macro-nutrient is needed in large amounts and gets used up fairly quickly.
Over a period of a few years, I reduced my tank lighting down to 8 hours (7 for the one tank) and I have avoided problem algae (which was an issue in some tanks, brush algae especially). I know the intensity is adequate for the plant species I have (higher-light plants will not grow in my tanks) so the balance has worked itself out. The increase in natural daylight (intensity and duration) entering the room in the summer caused me algae issues for a couple summers until I figured this out, and now with the windows heavily draped all summer that problem is gone. I mention this only becuase it shows how delicate the balance can sometimes be, and how little it might take to offset it.
Provided they have some rest for several hours of total darkness, plants don't really care how long the daylight is.
Byron.
