How to calculate aquarium lights?

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Barry Tetra

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I have this problem where I need to buy 3-4 lights (5watts) to make plants pearling.

Questions
1) I was wondering if there are the way to calculate how many watts to use per gallon?
2) I just started a new high-tech tank and was wondering if there is any light alternative to high tech lights.


@Colin_T @Byron @itiwhetu
 
37570772-C7E9-4C3B-BE3C-A206A88C958C.jpeg

I really rate this light I not an aquarium light but it’s a indoor growing light for yano tomatoes n stuff lol all 4 can be turned off and on individually and has 3 differnt settings to encourage photosynthesis
it’s far too powerful for my 500l tank and keep it on some of the lowest settings think it’s cost me £30 off Amazon lol
 
Lighting for terrestrial plants isn't normally as bright as light for aquarium plants because water reduces the amount of light that can get to the bottom of the tank.

I usually put my hand on the surface of the water under the light, and see if there is a shadow of my hand on the bottom of the tank. A decent shadow means there should be good light getting to the bottom.

Tall tanks need more intense light to penetrate the water depth.
Shallow tanks use less light.

Skylights work well when it comes to lighting tanks.
 
I'm not sure that's the good way to test it Colin. The fish store (espescially the plants store) wont let you test the stuff before you buy them.

Is there anyway like watts per liter calculation?
 
Light should be between 5000k to 6700k for best results keep on between 6-10 hours a day. Watts per litre/gallon doesnt really apply these days as light systems vary immensely with the introduction of LED. Intensity of light is measured in lumens.
20 lumens per litre is good for low light plants like ferns and anubius.
30-40 lumens per litre for medium grow plants like swords above
40 lumens per litre for high grow plants with red on them and dense carpeting plants.
 
Good question, Barry. It is bewildering. I've gotten pretty good at the TLAR method (That Looks About Right), but that doesn't help when making a purchase. @mbsqw1d is right about lumens, although I wouldn't measure it in lumens per liter or gallon, because so much depends on water clarity and the shape and depth of one's tank. I'd measure it in lumens at the substrate. I find that very few light manufacturers divulge output in lumens, anyway. For a good, practical rule, I'd say @Colin_T 's hand-shadow approach makes about as much sense as anything.

But @Barry Tetra is right; none of those help much when making a purchase, and experimenting can get expensive. Anybody else?
 
There was a formula with I think 1 watt per gallon was low light, up to 5 watts per gallon was really good light. However, that doesn't work as well with LEDs.

On my 4 x 2 x 2ft plant tank, I had 6 x 4 foot long fluorescent globes. Each globe was 36 watts. It was 216 watts on a 400 litre tank. The plants did well.

On tanks that are not as high, then more light gets through the water and the plants do better.
 

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