I have been running this way with my new setup for about 2 months now and have been pretty happy with the results. I do have a larger difference than you are proposing between dawn/dusk and midday - I am running about 10 PAR at dawn, 65 PAR midday, and 5 PAR at dusk. Once I got it dialed in to reasonable levels (was running midday too high at first), I have noticed very little additional algae growth. Now if I can just get rid of the BBA that grew during start up, I should be good.

If you are using an Android or iPhone, try downloading the Tropica application. If you are using a Med to High light and you are just starting up a new tank, the ideal is 6 hours a day for the first 3 weeks. After that you may increase the photo-period to 8 hours. On the 6th week, you could then increase it to up to 10 hours. Hope this helps.

As long as you do not have algae or plant health issues with a longer photoperiod, I would try for as much as 12 hours (more viewing time). I am currently at eight hours in a planted tank set up in January but now that my algae issues have subsided, I will soon increase the photoperiod. My established 600-gallon reef tank was a 12-hour photoperiod with no algae problems.

Mike

Like the responses above, it will depend on your tank. How tall it is. The nutrients in it. The plants, etc. A lot of different factors.

It's always good to pick a starting point and see what happens as each tank is different. 6-8 hours sounds like a good starting point, especially if you have the dusk like on before and after. You may get algae, you may not. You'd have to adjust accordingly. Good luck!

Wouldn't this depend on the types of plants, how much CO2 is running, lighting, etc? Seems like there wouldn't be a definite answer. Observation of your own aquarium is probably the best gauge.