Thank you. I may be able to suggest an alternative that should help here. First, the siesta approach is a response to the issue of CO2, so that is why for the plants it works. The period of no tank lighting in between two periods of lighting allows the natural CO2 to rebuild. So the result is that the tank lighting can be on longer (in total during the day) than otherwise. Some may not understand why, so I'll explain.
Light drives photosynthesis in plants. Without adequate light intensity (spectrum does enter in too) photosynthesis cannot occur. Once the light is adequate to drive photosynthesis--and every plant species is different--the plant will photosynthesize full out (assuming the other essential nutrients are all available). But as soon as some factor in this balance of light/nutrients is missing, photosynthesis slows and may even cease, depending. This botanically is known as Liebig's Law of Minimum, which says that plant growth is not governed by the amount of resources but rather by a minimum of some factor--the limiting factor. This in an aquarium is usually CO2.
Carbon (CO2) is one of 17 nutrients, but it is a macro nutrient. It is relatively easy to add most of the other nutrients. Some are present in fish foods, others in water changes, and in plant additives. When we rely on natural CO2, it can become depleted. This is because the plants in the aquarium, under usually much more intense light than what they would see in their habitat, and assuming we supply sufficient of the other nutrients, will be photosynthesizing more rapidly than in nature, generally speaking. So CO2 tends to be the first essential factor to give out.
There is a lot of natural CO2 in an established aquarium. The respiration of all fish and plants, and some species of bacteria, supplies CO2. But much more occurs from the breakdown of organics primarily in the substrate. During darkness, which for our purposes means when the tank lighting is off, the CO2 builds up. When the tank light comes on, the plants begin photosynthesizing and using this CO2. Depending upon the light intensity, the plant species and numbers, and the other nutrients being available, at some point the CO2 will be exhausted, or at least less than what is necessary to balance the light. Photosynthesis then slows. If the light intensity is not reduced, plants cannot continue to use the other nutrients because CO2 is lacking, and algae takes advantage. So the siesta allows the CO2 to rebuild, as plants are not taking it up during periods of "darkness."
So, back to your situation. I would reduce the period of tank lighting to one continuous period but fewer total hours, or you will almost inevitably see algae increase. I have experimented for months with my lighting, and it took a couple years but I got it to the balance between light and nutrients, at eight hours daily. One tank is seven hours because the tank is shallower and the same light is thus a tad stronger, so reducing the duration by an hour took care of that. I have the windows heavily covered during summer, as the additional intensity/duration of daylight in summer was enough to tip the balance and cause outbreaks of brush algae. It is all part of the balance.
