01-11-2011, 06:23 PM
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Welcome to Tropical Fish Keeping forum.
As previous member's responses have mentioned, the answer to your question depends upon several things. Plants need light in balance with available nutrients in order to photosynthesize (grow), and if any one essential is lacking, they can't. So the light duration must balance the nutrients. If light exceeds the available nutrients, algae will take advantage. Light should always be the limiting factor in plant growth to avoid this.
I would use a timer and have the light on during normal viewing periods, probably trying 12 hours to start with, and monitoring plant response and algae. If the latter increases, reduce the duration by one hour, and so on. We could go into more detail if we knew your light specs, plant species and number, and fertilization data.