# Lighting Hours - EI and Excel Tank



## Cloozoe (Feb 14, 2010)

Hi All,

Have a heavily planted, Excel and EI fed tank. I've researched lighting for such a set-up and everything I read suggests somewhere around 2 wpg or a bit less, but I've been unable to find any advice on _how many hours/day_ light should be on. Seems to me watt-_hours_/gal/day is the issue, not merely watts/gal; for example, if I ran 4 watts/gal for only an hour a day, net effect would presumably be an _under_-lit tank.

Currently I'm running 48 watts of T5 HO lighting with reflectors on a 26 gal, tall (21"), bow-front tank, for a total of 9 hours/day (4.5 hrs on, 3 hrs off, 4.5 on). Plants are doing fine and no algae other than reasonably minimal green-spot growth, or at least nothing that my Otos and Crossocheilus don't eat before it becomes visible.

Can I push the hours of light up a bit, (shorter plants are pretty slow growing, presumably due to depth of tank) or should I leave well enough alone?

Thanks!


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## TropTrea (Jan 10, 2014)

Plants need both a respiration as well as a photosynthesis period. In the ideal world these periods are roughly equal, or about 12 hours each. Shortening either of these periods of time to 10 hours is usually not an issue that is noticeable. 

However when you increased lighting the reduction in the photo period has a much more drastic effect. The plants can only absorb so much light for photosynthesis and any excess you give them at the moment cannot be stored for later use. So while your giving them 3 watts for gallon for 9 hours or 27 watt hours if they can only absorb 18 Watt hours causing the other 9 what hours to start burning the plants or if real lucky just get wasted. 

On the other end of the spectrum if they can only use 20 watt hours in 10 hours but your delivering them 1.3 watt hours for 16 hours for a total of 20.8 watt hours you could still be on the low end because they cannot use the lower level of light as efficiently as they can use an optimum level of light. 

Then the other factor falls in that they need the respiration period. So regardless how ideal the photo period is if they do not get at least 10 hours of near darkness there balance will end up being upset. While lengthening the dark period will not hurt them on this shortening it will. 

On a final note the watt per gallon guidelines were great when the most efficient lighting out there was the standard T-12 florescent bulbs with solid reflector. However today the rang of available efficiency of lighting systems used on aquariums ranges from about 75% of the efficiency of those old T-12 to well over 300% the efficiency. 

In your case not knowing the efficiency of the fixture I can only make a rough estimate. You have roughly 1.8 watts per gallon. If the tank is heavily planted then 10 to 11 hours of lighting should not be an issue there is possibility you could even increase it to 12 hours with caution.


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## Cloozoe (Feb 14, 2010)

Many thanks, TropTrea. Yep, I knew about photosynthesis vs respiration and also figured that 1000 watts for 1 minute was not likely to be the same as 1 watt for 1000 minutes for that reason and others; but with that said, it stood to reason that x watts for y minutes was not the same as x watts for 2y minutes.

As a follow up:

I run the 4.5/3-off/4.5 schedule purely so I can enjoy the tank mornings and evenings; nothing I've been able to find seems to support the hypothesis that the "siesta" reduces algae growth. Conversely, though, I have read that photosynthetic efficiency continues to increase for about 8 continuous hours before leveling off. If that were true, it would imply that 4.5/off/4.5 would _not_ equal 9 straight hours . True? Not true? True but trivial?

The fixture I'm using is a GLO (Fluval, IIRC?) incorporating a curved metal(?) reflector, that I've had for years. One Giesemann midday and one Giesemann aquaflora.


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## TropTrea (Jan 10, 2014)

Yes there is truth to this. But it has to also be considered with other things. When you look at nature the PAR rate is not constant. In the morning it starts to build slowly, it peaks at mid day then declines slowly till disk. Some plants will actualy shut down the photosynthesis operation at the peak in mid day to protect themselves from excess light. Other plants will cherish that peak.

Tests done with constant lighting levels show another interesting effect. As you noticed over time the photosynthesis increases even though the lighting level is constant. Then at a point in time it begins to decrease as the O2 levels in the plant reach beyond normal saturation. Dependent on the lighting level and the particular species of plant this can take anywhere from 4 to 14 hours but usually runs in the 8 to 12 hour range. 

In your case we are looking at roughly a 12 period of time, in which your giving them two 4.5 period of light. They are probably getting some incidental room light for the middle 3 hours so they may not get into the respiration phase. 

Yes the difference may be trivial but it is not as efficient as a single period would be. If you want to do the double lighting period I would consider the possibility of shortening one of those periods and added the time to the other period to get one period of time longer if possible. Your running 9 hours of light now but you could get some gain by spliting it to a 3 and 6 hour period rather than a 2-4.5 hour periods.


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