04-21-2010, 07:21 PM
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Am I correct in assuming that these are screw-in bulbs, and not fluorescent tubes? And are you using any plant fertilizer?
Plants generally manage with somewhere between 8 and 15 hours per day. The trick is the balance with the nutrients. If the nutrients run out at 10 hours, 15 hours of light will mean algae problems. In all my tanks over the years, it has taken a couple weeks to work out the exact duration that will benefit the plants but not increase algae. Plants have somewhat differing light requirements, but I would start out with around 10 hours. However, if algae begins to appear more than "normal," then the answer is to reduce the duration (or intensity should that be determined). Bear in mind that in newer tanks (up to 3 or 4 months) the biology is not yet settled and algae can come and go more frequently.
A timer is a good investment; you can buy them for lamps at hardware stores. Having the light come on and go off at the same time every day is better for the plants--and the fish actually. Both need about 10 hours of total darkness each night. Aside from this, you can have the light on whenever, obviously ensuring your normal viewing times fall within the light period. Make sure there is some light, whether daylight or a lamp, in the room when the light comes on and goes out; tank lights coming on or going out in a totally dark room will stress the fish quite a bit.