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Discussion Starter · #1 ·
Hi everyone, I have unique tank dimensions 36"x36"x12" and have been pulling my hair out trying to figure out what lighting I'd need. Anyway I purchased a Current USA with 1x 150W MH 10,000K & 2x 96W Daylights 6,700K. Since my purchase I've been told my lights would be underpowered for the lower depths, way to extreme, and about perfect and just about everything else you could say in-between.

To make a long story short I stumbled across this simple method to calculate light requirements. As of now I can't say for sure how accurate it is but I'm about to find out. Using this formula my light's should be almost perfect for an 11hr lights on day.

The Formula:
When calculating the amount of lighting you will need there is a general of thumb. First multiply the surface area of the aquarium by the distance from the light source to the top of the gravel. Then depending on the type of plants you desire multiply this by one of the factors given below.
Low light plants 0.08
Low to Moderate light plants 0.12
Moderate to Bright light plants 0.18
Bright light plants 0.27
This will give you the ideal watt hours of fluorescent lighting that you need. Divide this number by 11 and you now have the approximate total wattage of lights you need. Unfortunately this number may not be equal to what is available in bulbs so find the combination of wattage that will most closely match this requirement and adjust the available time to match the watt hour calculation.
Example: required watt hours is 1440, divided by 11, is 131 watts of power. since the closest is 3, 40 watt tubes we divide 1440, by the 120 watt total and we find we need 12 hours of lighting at this level.

Warning: A common mistake is to deviate greatly from the 11 hours of light to compensate for low or high wattage. If the light time exceeds 16 hours more wattage should be added to reduce this time, Or if the light time is less than 8 hours less wattage must be used to allow adequate time for photosynthesis.
When selecting plants also keep in mind that large center plants will shade the smaller plants under them and that higher light requiring plants should not be selected for small filler plants.
 

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That method is a joke, I'm pretty sure. The surface area times the depth is just the volume of the tank, the number of gallons. But, there is no simple way to determine how much light you need from the volume of the tank.

If you put a light above a one foot deep tank, the intensity of the light at the bottom of the tank, in the center is the same no matter if the tank is 10 gallons or 100 gallons. The difference is that the bigger tank will have very dark areas around the center, so you need multiple bulbs spaced widely apart to get that same intensity over the entire area of the substrate. It is the distance from bulb or reflector to the substrate that determines the type of light needed, not the volume of the tank. And, you get the same amount of light from a 24 inch long bulb as from a 60 inch long bulb of the same type. The longer bulb just lights up a lot more of the substrate.
 

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Discussion Starter · #3 ·
That method is a joke, I'm pretty sure. The surface area times the depth is just the volume of the tank, the number of gallons. But, there is no simple way to determine how much light you need from the volume of the tank.

If you put a light above a one foot deep tank, the intensity of the light at the bottom of the tank, in the center is the same no matter if the tank is 10 gallons or 100 gallons. The difference is that the bigger tank will have very dark areas around the center, so you need multiple bulbs spaced widely apart to get that same intensity over the entire area of the substrate. It is the distance from bulb or reflector to the substrate that determines the type of light needed, not the volume of the tank. And, you get the same amount of light from a 24 inch long bulb as from a 60 inch long bulb of the same type. The longer bulb just lights up a lot more of the substrate.
It's not figuring volume, it's going from the light itself to the substrate.

Surface area * Distance from light to substrate * value assigned based on plant requirement / 11hours?

I see your point on the center (focal point) intensity. This would be a negative factor for a light setup like I have with the MH in the center. But I think for tube style lights that go the width of the aquarium this could work well.
 

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That is saying that my 65gl tank needs around 170 watts for it's mid to low light plants, if I'm doing the math correctly (36 x 18 x 24 x 0.12 / 11) but you can see it's doing fine with only 78 watts T5HO. Doesn't mesh, there are too many other factors involved, light type, form factor etc. The same tank did great with 192 watts PC, but I was following the 3wpg guideline and hadn't started testing the lower threshold yet, and the tank ran into a lot of problems then.

For what it's worth, I think your MH is ok for 36", but wouldn't be surprised if you have green dust due to the 12" depth, front to back. Think of the light energy as an orb below the fixture, that MH is going to be pounding on the front and back glass. I would use a powerful linear fixture on your tank, with a good reflector designed to shoot light straight down, preferable T5 for the extra penetration. Judging by my 65, I would say the 100w range would be best for you, give or take, enough usable light for plants and less for algae, let the CO2 and ferts do the pretty work.
 

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Discussion Starter · #5 ·
That is saying that my 65gl tank needs around 170 watts for it's mid to low light plants, if I'm doing the math correctly (36 x 18 x 24 x 0.12 / 11) but you can see it's doing fine with only 78 watts T5HO. Doesn't mesh, there are too many other factors involved, light type, form factor etc. The same tank did great with 192 watts PC, but I was following the 3wpg guideline and hadn't started testing the lower threshold yet, and the tank ran into a lot of problems then.

For what it's worth, I think your MH is ok for 36", but wouldn't be surprised if you have green dust due to the 12" depth, front to back. Think of the light energy as an orb below the fixture, that MH is going to be pounding on the front and back glass. I would use a powerful linear fixture on your tank, with a good reflector designed to shoot light straight down, preferable T5 for the extra penetration. Judging by my 65, I would say the 100w range would be best for you, give or take, enough usable light for plants and less for algae, let the CO2 and ferts do the pretty work.

Well I tried. LOL, I've got mine up and running now. So far so good, my Swords have light pearling, Vals are pearling tons. Everything else appears healthy but no pearling. I'll keep you all posted as things settle in.
 
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