Light - how much do you really have?


Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida
Hi all,

I see a lot of questions regarding intensity of lighting and 'how much will I have' questions without speaking in terms of PAR or PUR but instead of WPG (watts per gallon).

So just to highlight how much I think we UNDER ESTIMATE the amount of light we provide, I did a simple measurement.


Tank: 45 gal tall 30X12X24 LXWXH
Light: twin bulb all glass light strip with built in reflectors
Bulbs: Verilux T-12 30watt for 60 total watts. Bulbs are in use for at least 18 months with no cleaning prior...
WPG: 1.33
Fixture is mounted 6 inches above the surface. The strip is centrally mounted.
PAR meter: Apogee PAR meter

I measured the PAR at just under the surface, mid-water, and substrate.

The ranges were as I moved the sensor back and forth at the same horizontal level (more or less).

Surface range: 30-60

Mid-water range: 15-35

Substrate range: 5-15

Please note also that shading from other plants also affected these ranges at the various levels.

I would consider this a 'low light' tank regardless of HOW the lighting was provided if the PAR was the same... I contend that most modern T5 fixtures will be quite a bit brighter indeed...esp as many folks mount them on short legs close to the tank.... I do not submit this as any sort of 'proof' just a suggestion that the light levels we have may be higher than we think they are.

Plus add that some use a short burst of MH lighting and it can really challenge you to provide optimal c02 levels....

Also to support that WPG is meaningless in every sense of the word as applicable to understand light levels.....

The L. aromatica was transferred from my c02/EI display tank about 6 weeks ago along with the blyxa and narrow java.

The aromatica still grows very densely but a much lighter shade as you can see from the video..I submit that this is due to loss of c02 and nutrient dosing..

So, stuff grows and well but more slowly...Maybe we don't need high light for aromatica and other stems????

Tank is stuffed full of fish the way I like it...The rasboras are going into the 180 later along with the cards and the majority of the platies will go to friends
and the LFS.....

I completely endorse buying, begging, or borrowing a PAR meter to really get a handle on the light you REALLY have...

I was blown away at first at the levels I was giving my 180...over 300 at the surface and 150+ on the substrate!!!!!!

Lights are now mounted 24" ABOVE the tank :)

I merely suggest that many of the 4-6 bulb T-5 fixtures may be more than needed.....

Spend your money on other high grade stuff that does not need to be your most expensive purchase...

Appreciate any comments.

vid link:
Last edited by a moderator:

Tom Barr

Staff member
Jan 23, 2005
So try a pair of T5, say a tek fixture for comparative purposes over the same tank.
Or a HQI MH's etc.

Tom Barr


Plant Guru Team
Lifetime Member
Jul 6, 2009
The Netherlands
Gerryd;52589 said:
I completely endorse buying, begging, or borrowing a PAR meter to really get a handle on the light you REALLY have...

I totally agree on that. It would save people a lot of issues they are having with guessing and assumptions about light.

Gerryd;52594 said:
Pretty sure I have a 24" 150 w MH but no T5.....

If you need T5 data I'm sure I can help....



Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida
Hi Dutchy,

I know you have T5's and a PAR meter, but the tank config and mounting height must be the same...

I will be comparing 150 w of MH vs 60 w of T12 over the same tank and mounted the same height...

I need to borrow a T5 fixture....

I bet the MH output > 250 PAR at the surface... takers??? lol


Jun 14, 2010
San Diego, CA
I have a 25G tall tank that measures ~ 24" tall
It has 1X55 W PC, with an AH supply reflector, the bulb has been in use for a little over a year, which puts it overdue for replacement.
It also has a 1X18W T8 that's newer.

Both bulbs are about 12 inches above the water's surface in a hood.

That puts about 3 feet from the bulb, and the bottom of the tank.

I measured between 2,800 and 3,000 LUX at tank bottom.
Using a very ruff conversion to PAR it's around 46-48, Far more than I expected.