don_kihotis said:
Plants do not utilize light for more than 8 hours per day whilst algae does. I also used to have my lights on for 12 hours a day and in combination with other mistakes got hair algae too. Now i have them on for 8 hours a day and i even think to go less than that.
As cowgirluntamed mentioned, there is much more to this than just the duration of the light.
In an aquarium, we create a very artificial environment to what aquatic plants receive in their habitats. Within a very confined volume of water, and water that is pretty much stable from water change to water change, we increase nutrients way beyond those in the habitat, and in many cases the lighting is more intense. There is room in nature for fluctuations, partly because of the vastness of the environment which can regulate or diverge this or that, something that is very limited if not even impossible in an aquarium.
The balance of light intensity/duration and nutrient availability is the key. Plants grow by photosynthesis, and this process is driven by light, primarily red but also blue. And the intensity must be sufficient for the plant species, and species have differing requirements. Provided the light is of sufficient intensity and spectrum, and all required nutrients are available, plants will photosynthesise full out. At the point at which something is no longer sufficient, be it light or a nutrient (and somewhat depending upon the specific nutrient), photosynthesis slows and may even cease altogether. We term this the Law of Minimum: plants grow up to the point at which something is lacking.
In terms of duration, plants need a period of rest, just as animals do, every 24 hours. This must be complete darkness. Provided there are several hours of total darkness, plants will function normally. The period of daylight that is of sufficient intensity to drive photosynthesis in the plant species can vary, provided all the required nutrients are available. I have had planted tanks run very well with 15 hours of tank lighting, and I have had them run with 7 hours of tank lighting (tank lighting meaning the "daylight" period of the brightest light). Obviously the nutrient availability varied between these two extremes; the plant species were much the same.
The goal therefore is to provide lighting of the correct intensity for the plant species, and have a duration that is balanced by the required nutrients. The plant load is also part of the equation. And the fish load and feeding, since this is a major source of nutrients, in addition to any we might add as fertilizers. If this balance is achieved, plants will do well and algae will be disadvantaged. As soon as this balance is no longer present, plants cannot use everything (be it light or nutrients) and algae will take advantage. Algae is not as demanding as higher plants when it comes to light and nutrients. "Problem algae" is due to the imbalance, and restoring the balance is the only way to safely and effectively deal with problem algae.
Byron.