Designing a Light source for a Fish tank:
Please note that this is not meant to be a tutorial, but rather a set of proposed instructions &
recommendations to arrive at a successful light source for a fresh water fish tank with live plants.
Deviations from these instructions are better left to specialists, who (I expect) know what they’re
SECTION II: Using LEDs
The best reason why you would like to change to LEDs is the short replacement time of the
With fluorescent lights you are advised to replace them every year. You may be able to stretch
that to 1½ or even 2 years, but that’s no comparison with the published Cree-range of 70%
lumen maintenance for 50 000 hrs, which - @ 8 hours/day - amounts to 17 years of operation.
Currently, the emerging LED market seems primarily aimed at supplying innovative and fancy
lighting in demand for home use.
Actually, no-one is as yet expecting tunnel farmers to start growing strawberries under LEDs.
The result is that – with the exception of some manufactures - technical information is very
sketchy, and difficult to use to design a practical light source for a planted fish tank.
If you do try, you might get involved into designs like this:
These designs are made with 5mm “super bright” LEDs operating at 20mA bias current,
generating about 60mW per LED or ≈ 30 Watt for the 462 LED fixtures. They create wonderful
displays, but they are under-powered and very expensive, and not specifically optimised - or
tested - to promote plant growth.
For this reason, we limit ourselves to the Cree – range of power LEDS, mostly because they
supply enough information to design a well-working unit.
If you decide to go this way, then you are recommended to research and plan the project
well, because you are otherwise very likely to fail.
Guiding parameters for LED lights are:
1. The allowed bias currents in mA.
2. The allowed power dissipation in Watt.
3. The PAR/Watt values at different bias currents.
Usually the manufacturer only publishes the luminous response either in lumen or mini-candela at the lowest
The PAR/Watt values are required to determine the effectiveness of the light unit at various settings. Unfortunately
that information is hardly ever published.
However, if the spectral response is known (as in the Cree-range) these values can be calculated.
4. The “Kelvin”-rating in degrees K.
As with the fluorescent lights, “Day-light” or “Cool-light” LEDs (5000 – 8300K) are expected to promote plant
growth best, although they may need to be softened with a few “Warm-lights” in the range of 2600 – 3700K.
Best estimated power ratio (Day/Cool-light to Warm-light) would be: 2 : 1
1. Water Surface Area:
You are required to calculate the surface area of the fish tank in m².
If you have a tank that is 60cm long and 30cm wide, then the water surface area is:
0.6m x 0.3m = 0.18m²
2. Light Intensities at the Water Surface:
This procedure uses the light intensity as distributed across and just above the water
surface as the light standard of the fish tank in:
One aspect we don’t need to worry about is reflector efficiency, because we don’t need a reflector. The light shines
down in a predetermined beam of about 180°wide. With care, all the generated light will hit the water surface.
3. Accepted Light Limits:
Lo: If you prefer a low-tech tank (no CO2) with low light plants (e.g.: cryptocorynes), then
the water surface intensity of the light source should be anywhere:
between 50 and 100 PAR/m².
Med: Medium intensity – where in principle you should be able to grow almost any plant
– is anywhere: between 100 and 250 PAR/m².
Hi: High intensity is anything: above 250 PAR/m².
4. How to determine the number of LEDs required.
To explain this we must look at a typical table in the LED Light-Spectra file.
This table applies to a Cree-xpg Cool-white LED.
With exception of the lumen value of 130 under “LUMEN” and the bias currents of 350, 700,
1000 and 1500 mA, all numbers are calculated based on info derived from the spectrum of
The 2 right-most column indicates the number of LEDs required to generate 100 PAR.
At low bias currents the LEDs generate considerably more than one PAR/Watt. However, we will see that in the
operational range around 1000mA this ratio is close to one, in which case either PAR or Watt can be used as
measures of intensity.
The formula is:
0.9 Watt LED PAR / Watt LED
The 0.9 factor covers the loss of light as a result of a junction temperature of ≈ 70°C.
The efficiencies indicate the percentage of the power dedicated to generating light. The remainder – 75% on average
– is turned into heat as shown in the right- most column, where the heat in Watt generated by each LED is shown.
When using LEDS, people are often recommended to apply the lowest bias current they can
afford. In this case, they would need to buy 66 LEDs to generate 100 PAR at 350 mA.
The lowest bias current gives the lowest junction temperature, the highest efficiency of
operation and the lowest heat generation, which is useful to secure a long light intensity
Unfortunately, these LEDs are expensive. At the moment of writing they are being sold in
South Africa for a minimum of R35 each, so it’s well worth considering this question carefully.
Obviously, going up in the bias range, you get into conditions where you can use less LEDs
as the power per LED goes up.
For this LED the recommended level is 1000 mA, in which case you need 34 LEDs for 100
Higher levels of bias-current – although theoretically permitted - could get you into trouble maintaining a low junction
temperature, which may force you to install additional cooling. The higher temperature may also cause a significant
drop in light intensity.
At a surface area of 0.18 m², the required intensities would translate to light ratings and
number of LEDs of:
Intensity PAR/m² m² PAR Bias-current [mA] Nr of LEDs
Lo-light 50 – 100 0.18 9 - 18 1000 3–6 8 – 16
Med-light 100 – 250 0.18 18 - 45 1000 6 – 15 16 – 40
Hi-light > 250 0.18 > 45 1000 >15 > 40
5. The Heat Sink.
Here you must certainly do some home work. Luck may be lenient if you fit a few LEDs on a
flat piece of aluminium, but if you go up on that, life becomes a bit more complicated.
LEDs get very hot and it is important to mount them on a fair size heat sink.
A good rule is that you shouldn’t burn your fingers on the fixture while the light is operating. If
that happens, then the LEDs are probably overheating, so be prepared.
But there are more subtle ways to follow:
1. Start with a small pilot project, enough for a small, lo-light tank with just a few LEDs
operating at 350 mA, but upgradable to a higher current at a later stage (An adjustable
current supply may be a good idea).
2. Mount the LEDs on a flat piece of aluminium sheet a few mm thick and as large as you
can fit above the tank. Screw the LEDs down to the aluminium sheet, applying a bit of
thermal putty so you can recover them easily later for alternative use.
3. Measure the temperature of the aluminium when the unit operates (this may require a bit of
4. If the aluminium sheet runs too hot, then drill a number of 3mm air holes (so-called: “vias”)
uniformly distributed across the sheet. It will dramatically improve the thermal convection
Unfortunately, if that doesn’t work you may need to buy a professionally manufactured heat
sink made from extruded aluminium profile with cooling fins – with or without forced air
cooling – and as big as you can afford.
In the end, it helps a lot if you can supply the heat sink manufacturer with the maximum
thermal resistance of your prospective heat sink.
How to determine the maximum thermal resistance of the heat sink.
Thermal resistances are like electrical resistances. If connected in series, you just add them up to find the total. If
connected in parallel you invert each of them, add them up and then invert the total.
R Rth:( sph )
Junction temperature of the LED: T junction Tambient Pheat th( j sp )
Rth:( ha )
, and so . . .
T junction Tambient Rth( jsp ) Rth:( sph)
Thermal resistance of the heat sink: Rth:( ha )
Please note that Rth is mostly quoted as Kelvin per Watt or K/W
also . . .
Temperature of the heat sink: Theat Tambient Pheat Rth:( ha )
Let’s assume that:
Tjunction = 70°C (arbitrary limit)
At the allowed junction temperature of 150°C, the LEDs lose 30% light intensity. At 70°C the loss is ≈ 10%, an amount
that has been taken into account.
Tambient = 30°C (common temperature above the fish tank)
Ptotal = 6 x 3.37 Watt x (100%-21%) = 15.9 Watt (see table), OR choose a design PAR level, e.g.: 18 PAR, then use
the 100PAR column and calculate the pro-rata number, i.e.: 18/100 x 34 = 6 LEDS, generating 6 x 2.65 Watt = 15.9 Watt
Rth(j-sp) = 8 K/W (manufacture’s information)
Rth(sp-h) = 1 K/W (manufacture’s information)
The thermal resistance between the LED solder point and heat sink, R th(sp-h), depends on the surface finish, flatness,
applied mounting pressure, contact area, and the type of interface material and its thickness. With good design, it can be
minimized to less than 1K/W.
Which will result in:
70 30 8 1
Thermal resistance of the heat sink: Rth:( ha ) 1.02 K/W is the number that the heat sink
It also follows that . . .
Temperature of the heat sink: Th 30 15 .9 1.02 46 .2C (hot but touchable)
There is an xls program available for load down, called: Heat Sink Formulae. It uses these relations and will allow you to
combine two types of LEDs from a selection of six Cree- LEDs, providing you with the thermal resistance of the required
heat sink and its final temperature.
6. How to mount the LEDs (more permanently).
1. The LEDs must be equally distributed across the heat sink, but at least 2.5 cm apart. It is
a good idea to mark the mounting positions.
2. Clean the surface with a suitable solvent to remove any dirt and grease.
3. Mount the LEDs on the indicated spots with a two part thermal epoxy. Use only a small
thin coat and gentle move the LED around on the spot before pressing it down, but be
careful because it dries in 5 minutes, so mix small portions.
7. Connecting the LEDs.
This is how it should be done.
The cross-connecting is a way to maintain some form of even heat distribution in case one of
the circuits breaks down.