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Using PAR instead of WPG to Measure Light Level in a Planted Tank


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  • Regular Member

The article below is meant as a very basic primer on using PAR instead of WPG to measure light level in a planted tank.

Using Watts per Gallon to Measure Light

“Watts per gallon” (wpg) has long been the way we measure low, medium, and high light in an aquarium. At the time this guideline came about, T12 bulbs were the primary lighting source in an aquarium. With the introduction of several newer, more efficient types of bulbs including T8, T5NO (normal output), T5HO (high output), and even LEDs it becomes difficult to use the old wpg guideline because these bulbs are not comparable in terms of their output.

The newer bulbs are narrower

T12: 1.5”

T8: 1”

T5: 5/8”

The smaller bulbs are more energy efficient (meaning they use less electricity to run) and also provide a higher output of useable light for the same wattage. Other advantages to these newer, smaller bulbs are that more can be fit into a fixture because of their small size, and the extra room created by smaller bulbs also allows for individual reflectors to be placed around each bulb in the fixture, which significantly increases the amount of light being directed at the aquarium.

So, a 24 watt T5 bulb is going to use less electricity and provide you with more light output than a 24 watt T8 bulb. Because of these differences, it becomes very difficult to determine how many wpg you need because these bulbs are not comparable.

The other issue with using wpg is that there are many factors in an individual aquarium that are going to affect the amount of useable light that actually reaches the plants. This includes the depth of the tank, the height of the fixture, and the quality of the fixture being used (we cannot even compare across the same bulb type easily because the quality of the fixture and reflectors used is going to significantly impact how much useable light reaches the bottom of the tank).

If we can’t use WPG, how do we measure light?

Recently, the use of photosynthetically active radiation (PAR) has been proposed as a more efficacious way of measuring light level in an aquarium. Lumens per square meter (LUX) is a standard measure of lighting intensity. It is used to measure how bright a particular light appears to the human eye in a standard indoor (or outdoor) setting. So, if we want a brighter light for our kitchen, we might choose one that has higher LUX. PAR on the other hand is a measurement of a particular band of wavelengths (400-700 nanometers) that is useable by plants for photosynthesis. PAR is measured in micromols of photons per square meter per second.

Essentially, PAR can be used to measure exactly how much useable light is available to plants. The amount of PAR available is effected by things like depth of tank (the same light fixture is going to provide higher PAR at 12 inches of depth and lower PAR at 30 inches of depth for example), height of the fixture, quality of reflectors on the fixture, and even where in the tank you are measuring (a back corner of the tank for example may have lower PAR than the center.

How do we measure PAR?

The issue with using PAR as a measurement of light in a tank is that it is not a simple calculation like wpg, and because this way of looking at lighting is relatively new, there is not a ton of PAR data readily available for light fixtures. PAR can be measured using a PAR meter, however these tend to be more expensive than most hobbyists are willing to invest. There are good guides to DIY par meters out there if you search around. You should be able to build one of these for under $100.

Luckily, there are several people in the hobby working on collecting PAR data for different fixtures, and some companies are beginning to provide this data to consumers as well. There is still a huge gap in the data at this point, but one hobbyist who goes by the username Hoppy has been instrumental in providing information on the use of PAR and data for specific light fixtures and bulb types.

Here are some of his graphs that you may find helpful below. The PAR can be divided or multiplied by the number of bulbs if your fixture has fewer or more bulbs than the one listed (for example, if you have a 2 bulb aquaticlife t5ho fixture and the one listed below is the 4 bulb fixture, just divide the PAR by 2).






What is Low, Medium, and High light?

According to Hoppy (who created the graphs above):

Low light - 15-30 micromols of PAR - CO2 is not needed, but is helpful to the plants
Medium light - 35-50 micromols of PAR - CO2 may be needed to avoid too many nuisance algae problems
High light - more than 50 micromols of PAR - pressurized CO2 is essential to avoid major algae problems

As Hoppy notes, these guidelines are subject to change and interpretation, as this way of measuring light in an aquarium is still very new and we are still learning. However, this is the most current guideline at this time.

Lighting and balance in the aquarium:

Now that you have a general sense of what light level you are working with, you need to determine if it is appropriate for your particular setup. In a planted tank, there is a delicate balance between amount of light, ferts, and co2. These three points need to be in balance or it will lead to problems. As light increases, plants demands for co2 and ferts also increase. If you are unable to provide adequate amounts of these, your plants will suffer and algae will likely flourish.

In determining how much light you want/need you need to consider what type of plants you are interested in keeping (some require more light and co2 than others to flourish) and what type of tank fits with your lifestyle and budget. High light/high tech tanks tend to be more expensive to run because you need to invest in good light and co2. They also tend to be more labor intensive because you need to dose ferts regularly and plants will grow at a rate that requires frequent trimming and maintenance. But high tech tanks allow you to keep a wide variety of plants. In a lower light tank plants require less co2 and ferts because their growth rate is slower. A low light tank reduces the need for added co2, fertilization needs are lower (and depending on bioload ferts may be provided by fish waste alone or a light fert dosing regimen), and tank maintenance in terms of trimming is reduced. A low light tank also reduces the types of plants you are able to keep successfully.

This is as far into this topic I will go because this is a whole other article in itself, however it is something to consider carefully when picking out your light fixture.

How to Reduce Light:

Too much light is a common problem once we begin looking at PAR instead of wpg. If you find you have more PAR than necessary for your setup there are a few ways to reduce the amount of PAR without getting a new fixture:

1. Raise your fixture

- Having a raised fixture is actually desirable for all lighting situations because it allows for a more even spread of PAR throughout the tank

- You can use the charts above to determine how much you need to raise your fixture in order to get a desired light level. Many fixtures have hanging kits that can be bought online.

- Lights can be hung from the ceiling, by building (or buying) a bar that attaches to the aquarium stand, or by using plant hanger hooks screwed into the wall. Please do your research well before deciding how to hang your light. A light fixture falling into a fish tank is something you want to avoid at all cost.

2. Use fiberglass window screen

- According to Hoppy, one layer of window screen will reduce PAR by 40%, two layers will reduce by about 64% http://www.plantedta...ad.php?t=114756

- Window screen can be bought cheaply at any hardware store

- If you have a glass top you can lay the screen on the glass top below the light, or you can attach with tape to the bottom of the fixture.

3. Use plants/floaters

- Floating plants can also be used to reduce light. Without a PAR meter it is difficult to know by how much, but you can easily adjust by adding or taking away floaters depending on how your plants are doing. The other issue with floaters is that it can be difficult to get an even spread. They may all congregate at one area of the tank depending on the current, significantly reducing PAR in one area and not others. Using plants can take some trial and error.

- Small floating plants like duckweed and frogbit can be used along with larger stem plants such as wisteria.

4. Reduce the number of bulbs you run

- If you have a multi bulb fixture you may be able to run it with fewer bulb, but this is fixture dependent. This is not an ideal fix, but may work if the others listed above are not options

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  • Regular Member

Please let me know if you guys have any additional information you would like to add to this article or any corrections to information contained in this article. I am still learning and this is by no means a comprehensive article ;)

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