Originally Posted by Nyxxi
thanks guys. If i am reducing the light how much can i reduce it and not effect the lotus in the one tank. I know my crypts and anubias will be fine but i have lost several lotus plants before i increased the lighting a while back. The lights are older and i was using them for growing some terrestrial plants at one point but they haven't dulled or anything what effect does older florescent tubes have on an aquarium so long as they're not burned out.
Light has to balance the available nutrients or plants cannot photosynthesize (i.e., grow). As soon as something is missing, be it adequate light or any one of the 17 nutrients plants need, they will slow down and may even stop altogether. If a nutrient that is essential is no longer available and light remains, plants can't use it and that is when algae takes advantage. Algae can also use poor light that plants might have to struggle under if they can manage at all. Which brings me to the age of the tubes.
As fluorescent tubes burn, they weaken in intensity. And it is quite rapid. If the tubes are the standard T8 (the "T"refers to the diameter in 8ths of an inch, so a T8 is 8/8 or one inch diameter) they should be replaced every 12-18 months. Every year would be better. Even then, they are considerably less bright than they would be new. When you are used to the tube day after day, you don't notice this; but everyone who replaces a tube is always aware of the brighter light.
Sometimes algae will suddenly start to increase, and it is often because the light has lost so much intensity it is too weak for the plants. Algae can use any light, at any brightness; plants cannot. I noticed this in my tanks last year; brush algae began increasing, and I checked and saw that my tubes had been running for 18 months. I replaced them; the algae immediately stopped. This is one reason you should never leave tubes until they burn out; they are far too weak by then.
As for the duration, it depends upon the plants and available nutrients. As I said earlier, everything has to be available in balance. Some nutrients occur in tap water (and come in with each water change), some in fish food that end up in organic waste in the substrate. Sometimes we have to add some via liquid fertilizer or substrate tabs. One very important nutrient is carbon, available to most aquatic plants as CO2 (carbon dioxide) although some can use bicarbonates (in harder water). CO2 is often the first nutrient to run out. How long do you have the light on now? And are you using any fertilizer? This will help us suggest a suitable reduction time.
Last, but certainly not least, welcome to Tropical Fish Keeping forum; I should have welcomed you previously, but didn't notice you just joined.