01-24-2011, 01:26 PM
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This "siesta" approach is frequently recommended to eliminate algae problems. How it appears to work is that during photosynthesis in the daylight (= tank light on period) plants use CO2 which in a low-tech (natural) system becomes less and less during the daylight period. Inserting a period of 2-3 hours "darkness" allows CO2 to build up again, so there is more of it for the next "daylight" period. It is said that this does not affect higher plants, provided it is within reason, but algae somehow finds it more difficult.
I say within reason because the plants need that total darkness period, which most authors suggest is around 10 hours daily. This is complete darkness, no daylight and no room lights. The fish also need this to rest up their systems. The usual siesta period is recommended to be 2-4 hours, with 5 hours minimum of light on either side. Which leaves 9-10 hours for the total darkness.
Kym's point about the effect of mini-days on fish is something that I worry about, and so far I have not found any reliable data. I have refrained from trying a "siesta" approach myself for this very reason. My fish are first, the plants are second, so I will not adjust something that may be acceptable to the plants when there is even a slight chance that the fish will be negatively affected. One must remember than many of those who advocate a siesta method are primarily plant-oriented hobbyists.