Lighting for Planted Aquariums (watts per gallon, LSI, lumen, lux…)

Having setup a planted aquarium and finding that my plants were not doing too well inspite of having 03 CFLs in an 80 liter tank, i realised there must be something fundamentally wrong. Assuming that  the light was sufficient, I set out of check the substrate, fertilizer etc etc. When those did not help either, came back to the fundamental question on light. Wearing out my finger hitting the Google search button on the internet and after reviewing many article on the subject, I thought i would share the result of my research on Aquarium lighting for Planted Aquariums which may benefit others struggling like myself.

Watts per gallon – this is now an outdated measure, i guess this was more relevant in the era of Candescent bulbs, however with so many different high efficient lighting system now available, watts per gallon just does not make sense anymore. So what makes sense now….

Lux or luminous power per area make more sense now. Read all about lumen and lux here.

So how does it work, you basically need to calculate Lux per square inch of your tank (LSI), here is how …

1. Every bulbs, CFL, LED fixture now comes with a Lumen rating to it. e.g. a 15 W CFL has a Lumen rating of 1400 lumens. So total up the lumens from all lights in your tank. I have 2 x 15 W and 01 x 18 W CFLs so that would be  2 x 1400 + 1 x 1250 = 4050 lumens

2. Next, get the surface area of your tank in square inches. My tank is is 1′ x 2′ = 12 inches x 24 inches = 288 sq inches

3. Divide total lumens by surface area = 4050 / 288 = 14.06 lumen per sq inch i.e. the LSI for your tank.

Now compare the figure from #3 above with the chart below to see how you are doing :-

  • Low light :  12-17 LSI
  • Medium light :  20-25 LSI — you can grow most of the aquarium plants available.
  • High light :  28-32 LSI — you can grow all aquarium plants.
  • Very high light : > 35 LSI — same as high

So as you can see even after 03 CFLs in a small tank, i am still very much on the lower side

5 thoughts on “Lighting for Planted Aquariums (watts per gallon, LSI, lumen, lux…)

  1. Figured that the current LSI was too low for good plant growth, did some research in the market and bought two 36W Philips CFLs @2900 lumens each. After some drilling and electrical work on the tank hood and am now at 20.14 LSI vs 14.06 earlier.

    Did this a week ago, the plants are doing much better, however the water temp is up by 2-3 degrees (around 31 degree C even though the electronic choke / basalt were fitted outside the hood / tank). The amazon are doing fine, but not the moss :-(

    Am gonna try to fit a small cooling fan inside and see what difference that makes ;-) , will keep you posted.

  2. Hi there, very nice article, and very nice explained, just have a small question, what about the spaces between each rank??? I mean 18 and 19 to reach the medium light???

    Thank you very much.

  3. Hi,

    Found your blog while searching for the same.

    Just wanted to let you know that, you did not consider HEIGHT of the aquarium while calculating the surface.

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