What is High, Medium, Low light in PAR anyway?


Junior Poster
Sep 3, 2007
Hi Everyone....

One thing Tom has been on about for a while now is that people will measure all kinds of things in an aquarium, but not light. So that seemed very reasonable to me and I splashed on a PAR sensor.

Only problem is, no-one seems to be saying what constitutes high, low or medium light. The values of course will differ depending on where you put the sensor. I put mine at the bottom of the tank, straight under the light, as close to the middle as I could without obscuring it. This makes the most sense to me because it takes some account of the depth of the tank, which measuring at the surface will not.

To look at it another way, for a given light source, a 12" deep tank might be classified as "High light" whereas a 24" tank could be called "low light". This would be taken into account if you measure by Watts Per Gallon (whatever that might mean), or measured PAR at the bottom. But if you measure PAR at the surface it will be the same for both tanks, even though the light levels in the 2 tanks would be quite different.

As for my setup I have 2 36W T8 tubes in a simple reflector over a 18" deep 50 Gallon Tank. I took it apart and replaced the ballasts with electronic ones. This allowed me to "overdrive" the tubes to get more light of them by using ballasts meant for 4 tubes rather than 2.

I get a PAR reading of 50 through the day. If I turn overdrive off this drops to about 32. So its useful already to know that I am squeezing an extra 60% or so out of my tubes with overdrive. So I suppose therefore I would have gone from "1.2 Watts Per Gallon" to "1.8 Watts Per Gallon"

But then I am left wondering - what does a PAR of 50 at the bottom of my tank mean? Medium? Low?.....


Tom Barr

Staff member
Jan 23, 2005
Say 50 micomol or less, 120 or less, 200 or higher. This at the tip of the plant, not the bulb or the surface of the water etc.

They are arbitrary measures, but are about what many consider from experience to be low, med, high etc.

Tom Barr


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
We have little control over the vertical distribution of light intensity in an aquarium. If the light is right at the top of the tank, the intensity will be much higher near the top of the water than at the substrate, but if the light is mounted high above the tank, the difference will be a lot less, and both intensities will be a lot less too.

So, I think we should rate our lights by PAR at the substrate, which also corresponds with where the light demanding ground cover plants live. If I have 50 micromols at the substrate, with the light 20 inches above the substrate, I will very likely have 200 micromols 10 inches higher in the water.

I'm still confused about high and low PAR values though. My 50 micromols at the substrate seems to grow plants (and algae) as if it is higher than low light, but not really high light. And, in watts per gallon I have 1.2 of PC AHS light.