12-03-2010, 12:27 PM
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I'll just expand a bit on the advice (correct) already mentioned.
Plants need a sufficient period to photosynthesize (which is how they grow/live). Nutrients must be in balance, or the plants cannot use the light. That is when algae takes advantage of the "excess" light. So we aim to ensure there is a balance during the period the light is on. This period can vary from as little as 6 hours to as much as 15 hours. Provided the plants (and fish) have a sizable period of total and complete darkness, usually 10 hours is recommended, they will manage with the period of light whatever it may be--but provided the nutrients are sufficient to balance the light period.
When these periods of light and dark occur is of no consequence to either the plants or the fish. But the point made by jeaninel is important--consistency. Plants and fish will be better with a regular stable schedule. My tank lights are now on for 10 hours daily. Years ago it was 15 hours. I am currently at 10 hours because any more and brush algae increases. And one must remember that light in the room, whether daylight through the windows or artificial room lights, also contributes. The "dark" period should be absolute darkness; for most of us this is late night to early morning.
Having your lights come on mid day and going off in the evening so you can be home to view the tank is fine. Using a timer is the best way to achieve regularity. When you feed the fish also has to be within the period of light. When I worked and left home around 7 am before it was light outside (in the winter), I fed the fish when I came home in the evening around 6 pm. They got used to that schedule, expecting food every evening but never during the day. You want to feed them when the light is on, and after it has been on for at least 20 minutes so they are accustomed to the light, and not later than an hour before the light goes out.
As for the length of time, that depends upon your aquarium: the biological balance is different in each aquarium, so sometimes it takes a bit of experimenting to find the right period. I usually start out at 10-12 hours, and as the tank becomes established over the first few months, I monitor plant growth and algae. Remembering that algae frequently appears during the first couple of months as the water is still adjusting biologically, so this is normal. But once the tank has been running for 5-6 months, any increase in algae means there is too much light. Assuming the intensity is adequate, the duration can be shortened by an hour. Then if this is insufficient, another hour. In summer with longer days and brighter daylight, algae can appear more, so perhaps less light in summer (an hour less). It depends upon the algae.