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darkness period + extended photo period

>I finally set up my 65 gallon 1st proper plant aquarium. Yay!
>I will have the lights on for 12 hours per day, correct? why do some people
>have a 10 hour photoperiod for a tropical tank?

"Tropical" covers areas that extend quite a way from the equator, so day
length does vary.  On top of that, In the early morning and late afternoon,
light is coming in at such an angle that it is probably not very useful for
photosynthesis.  Generally, people who use the brightest light levels
usually find that it is easier to balance the system using a shorter photo
period, while people who use less than optimal lighting might want to
slightly extend the photo period.  Generally, lighting that falls between
10-14 hours per day is in the right ball park, you need to find what works
best for your particular set up.  

Beyond 14 hours, most higher plants stop photosynthesizing even in the
presence of good light, while algae just keeps on going.  This can give
algae the upper hand on nutrient fixing.

>When the lights are off must the tank be pitch black? Is it ok if the tank
>lights are off but the room lights on? eg at night? will this small amount
>of light reaching the plants make the plants photosythesize weakly or will
>they respire as if it was dark? i supose the ambient lighting should be no
>problem as its probably no brighter then full moon light?

Room lights, or even ambient daylight that does not fall directly on the
tank is usually not a problem.

>Also is it ok say once a week to have the lights on for a few extra hours
>(3 to4) when entertaining? will this cause harm to the plants?

Probably not.  But if I were entertaining that often, I would probably
adjust the light schedule to coincide with my schedule better.  I have
young kids, and find that most of the time I have available to enjoy (and
work on) the tanks is in the evenings.  Therefore, my lights turn on at 11
AM, and go off at 11 PM.  On the few occasions that I will want the lights
on later than 11 PM (or earlier than 11 AM) I simply spin the timer dial
around and turn the lights on.  No harm done.

Karen Randall
Aquatic Gardeners Association