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Getting correct light from a hanging bright light

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I am just now setting up my first freshwater tank (36 gallon bowfront). I read several books and bought my equipment, including a MarineLand LED fixture specifically designed (6700 K red+blue) for freshwater planted tanks. (I have MarineLand LEDs on my reef tanks and I love them!). Then I discovered this wonderful forum and learned that I have to do a major modification of my plan.

I definitely do not plan on CO2, and several experts in another thread said that I therefore need to limit my light to 30 PAR max. The fixture I bought is 172 PAR at 12 inches. Yikes! Luckily, it will be fairly easy for me to suspend the light well above the tank to disperse its output.

But here's the question. How do I know how high? I hesitate to spend $300 on a PAR meter that I'd use once. Are there any good rules of thumb? Or can I just experiment and tell from the appearance of plants BEFORE I do major damage to them?

Thanks for any advice!

Tim
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I might were it me,,start with the light hanging eight inches above the surface of the water for six to eight hour's per day ,and observe growth of the plant's.
As you have noted,,can alway's lower or raise the light further.
Would be sure to provide some type of fertilizer such as macro-micro package from..aquariumfertilizer.com.
Would add this once a week minimum, one day after water change.
Several other places to purchase wet or dry fertilzer's perhap's, but they are needed for good growth low tech non CO2 ,or high tech CO2 injected.IMHO
Add all the plant's you can afford from the get go,and give them time.(week's).
 

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Discussion Starter · #4 ·
Signs of wrong light

Here is a real newbie question, a little more specific than my last. I've been perusing these forums for an answer, and here is what I seem to be seeing. Is this right (for a non-CO2 tank)?

If the light is too low, the plants will grow too slowly, or maybe even die back. I will get an early hint of dieback because areas of the leaves will develop more than the usual quantity of brown spots.

If the light is too high, the plants will do fine until excess algae growth on their leaves causes problems.

Am I on the right track here? Thanks!

Tim
 

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You shouldn't have to hang them really high. You have to realize that the par drops off considerably with height. That 36 gallon bowfront is like 21 inches high not 12 inches so you won't be getting all that par. Even if you have 3 inches substrate that still 18 inches. I would see what the par levels are at 18 inches before raising them so much.
 

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You can buy a lux meter, a light meter that measures light intensity in lux instead of PAR units, for about $15 on Ebay or Amazon. Its sensor will not be waterproof, so you will need to use it only in air. Then you can temporarily hang your light above the floor, and measure the light intensity in lux at various distances to determine at what distance you get whatever intensity you want. 30 PAR is about 2100 to 2250 lux, so you can adjust the height of the light above the floor until you read 2100 to 2250 on the meter, then measure the distance the light is above the floor. That is the distance your light should be above the tank substrate to get 30 PAR. (With the accuracy you need, not exactly.)
 
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