09-11-2011, 12:34 PM
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First off, welcome to Tropical Fish Keeping forum.
Now to the light. T5 presumably means HO (high output), which is considerably more intense light that regular T8. You haven't mentioned the number of tubes or their size, so I can't provide any guidelines; nor have you mentioned CO2 diffusion, and this has a considerable impact. So for the moment I'll leave the matter of total photo-period (duration) until I know these factors.
With respect to the "siesta" approach as it is commonly referred to, I believe you will need to adjust the "on" periods a bit. Plants need sufficient time to use the available nutrients in photosynthesis. Most sources advocating this method suggest 4-5 hours on, 2-3 off, then 4-5 on. Here again, the nutrient availability has a significant impact, particularly CO2, so knowing that will help; without sufficient nutrients available, the plants cannot use the light, and algae will take advantage.
There is also the matter of the fish. Plants can adjust to a "siesta" system, that we know. But the impact on fish is as yet not fully understood. I always address increasing algae issues by reducing the photo-period, and I have kept it continual every 24 hours solely because I am not certain of what long-term effect a siesta method might have on the fish. Biologically fish expect 10 hours of daylight and 10 hours of complete darkness during each 24-hour period [dawn and dusk being the other 4 hours obviously]. There are ramifications to this that I won't go into here. I have searched around, but so far not found any source that specifically addresses the issue. The ambient room light is of paramount importance. Most aquarium fish are forest fish that occur in dimly-lit streams with minimal or even no direct sunlight. Provided the ambient light is sufficient to still be "day" to the fish, the tank lighting could be considered direct sunlight between clouds or shading by forest canopy or something. This should pose no issues, but I am only guessing.