# Lighting watts per gallon?

##### Well Known Member
So I was researching lights and found on a different forum someone said
low light is considered 1 watt per gallon, medium 2, high 3 and over
So I am concerned because I have 55 gallons and 32 watts. Could this be why my plants arent doing well? The Java ferns are doing okay, some are growing roots. The Anubias arent doing well though and I want to add more plants. The fixture I have only has room for 1 bulb, so I am thinking that I need to get a new fixture that will hold 2 of these bulbs? Or was this person wrong and my light is fine? Its a 6500K T8...

#### Dom90

##### Fishlore VIP
I read somewhere that it should be 1-2 watts per gallon, but that also the PAR matters... Dadio may be able to help you out.

##### Well Known Member
Here is the packaging label for my light.

Sent from my SM-G900R4 using Fish Lore Aquarium Fish Forum mobile app

Okay I found this for figuring out the PAR value..

take the watt rating, multiply it by the PAR factor (1.0-1.4) and get the PAR rating at the source. Then we multiply by the reflector factor (~300%) and divide by the light loss at the surface (40%). Then we have a pretty decent estimate of the amount of PAR actually getting into the tank. This calculation has proven itself to be off by as much as 10% which makes it unscientific, but still relevant to the average hobbyist.

The PAR of light actually in the tank bounces around a lot, but for all intents and purposes, it pretty much gets reduced at a steady pace. (Inverse square law). So, to use our PAR value, we need to see how much is reduced for each tank size.

An optimal "low light" tank is going to need about 50 PAR at the substrate level.
An optimal "moderate light" tank is going to need about 100 PAR at the substrate level.
An optimal "high light tank" is going to need about 150 PAR at the substrate level.

Therefore, (because of the inverse square law)

12 inch tank: low -80 PAR at the surface
moderate -166 PAR at the surface
high -250 PAR at the surface
24 inch tank: low -166 PAR at the surface
moderate -333 PAR at the surface
high -500 PAR at the surface
My tank is 21 inches tall.

So 32 * 1.0 = 32 * 300%= 96 / 40% =240 ? I think that its supposed to be * 40%, which would then be 38.4 PAR +/- 10% would be from 35-41 PAR. IDK how to do the inverse square law or what number im supposed to be looking at, the 240 or the 38.4...

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##### Well Known Member
Okay now I found a post that lays it out a little easier to follow...

Watts (130) X PARvalue (1.4 b/c good light) = 182 PAR

182 x 300% b/c good reflectors = 546 PAR at the source

546 - 40% scatter b/c of the surface = 328 PAR at the top of the tank

328/2 at one foot = 164 PAR @12"depth
164 / (1/4) = 123 PAR at the substrate b/c inverse square law.
So subbing in my numbers I get
96 PAR at the source
58 PAR at the top of the tank
28.8 PAR at 12"
7.2 PAR at the substrate

I hope I did that wrong because that doesnt seem anywhere near what I need if I am understanding correctly....

#### Dolfan

##### Fishlore VIP
Watts per gallon is an outdated method that does not factor in depth, height above tank that light is mounted, quality of reflectors, etc. So don't even worry about it.

PAR on the other hand, is a much better way to judge light in our tanks. Here is a link that explains PAR...

It's a long read, but well worth it, as it masterfully explains how PAR works. In it you will notice where he estimates the amounts for different levels of light....

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 for your case, I have never seen that equation before that you used, but it seems like it may be a rough estimate. If you have 7.2 PAR at your substrate, that is too low for plants. You need 10-15 for even the lowest light plants like Java Fern and anubias. For a 55 gallon tank you definitely need something with 2 bulbs. This would fix a lot of your problems.

With modern technology, I would highly suggest switching over to an LED fixture. You will save money with lower electricity costs, less heat output, never have to replace bulbs or ballasts again (bulbs need to be replaced every 6-10 months in planted tanks or they age and lose their benefits). You will have a little higher investment upfront, but it pays for itself in the long run.

I highly recommend the Finnex Planted Plus LED's. They are perfect for low light tanks, and work very well. I'm assuming you have a 48" tank, so you would need their 48" model, which is around \$130. Here is a link...

That fixture would only use about half the electricity of a standard 48" dual fluorescent bulb light fixture. They also work with any \$5 timer from Home Depot, Lowes, etc.

Hope this helps.

##### Well Known Member
Thank you Dolfan!

That is what I was afraid of. This would explain why my plants arent doing too great. I guess my next investment will be a new light fixture.

#### Dom90

##### Fishlore VIP
I would invest in a LED of the right spectrum, bigger initial cost BUT cheaper energy costs in the long run. That energy bill adds up! lol

##### Well Known Member
I just looked up the wattage and it seems to be 35.6 watts. Which is just a little more than what I have now. I wonder how that gives so much more PAR rating lol. Yes it will be cheaper electric rates than getting a fixture that houses 2 of the bulbs I got. It would take several years to pay for itself in that respect, but compared to the alternative, I guess its on the only way to go. I wish there was 2 plugs, one for day and one for night because I got a light timer to turn them on (10am) and off (8pm).. I guess I just wont use the moonlight.

#### Dolfan

##### Fishlore VIP
LED's are much more efficient then regular fluorescent bulbs. You can replace a standard screw in socket bulb that normally uses around 20 watts, with an LED bulb that uses 5 watts, and they will be about the same brightness.

Your current fixture uses 32 watts for one bulb, so if you got the right size fixture it would have 2 bulbs for a total of 64 watts and even then it would be very low light depending on your reflector. But the LED I linked uses 35 watts and is a bit brighter, hence why LED's are the wave of the future. Save electricity costs as well as bulb/ballast replacement, which many people forget about, Those 48" bulbs costs at least \$5-10 each depending on what you get. Replace those once or twice a year and it adds up.

You will be very happy with a Finnex Planted Plus if you decide to go that route, it may be a bit more up front, but I promise you will be happy with the look. Your plants will thank you too, haha.

I run both the full regular lights and the moonlights during the day, and then all off at night. Your fish don't need any light at night. I run mine on a split schedule which is supposed to help with algae and supply more CO2 to plants as they need it. So I run mine from 9am - 1pm and then back on from 6pm - 10pm. This way also allows for me to enjoy the tank more at night while I'm home and able to see it.

You are right though, that is really the only complaint I have ever heard about the Finnex Planted Plus, is that people wanted separate power supplies for separate timers as you were wanting. Overall the fixture is pretty cheap, when you compare around to other LED fixtures designed for planted aquariums. So I guess we can't have everything.

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#### Dom90

##### Fishlore VIP
Well some website compared the energy costs of a T8 and LED over 50,000 hours and the savings with LED were huge.

##### Well Known Member
Well some website compared the energy costs of a T8 and LED over 50,000 hours and the savings with LED were huge.

I just did this the other day so I have a spreadsheet set up already and I plugged in 64 watts @ 50,000 hours and 35.6 watts @ 50,000 hours and the costs were \$416 and \$231, respectively. That difference is about the cost of the light. So over 50,000 hours (on for 10 hours a day), 13.6 years later, it will have paid for itself lol. But yes, I get what you guys are saying. I just have to wait to get the money to do it. Hoping in a couple weeks I can get a nice paycheck and do this, get more plants, and start up my 20 gallon.

#### Dom90

##### Well Known Member
Those are metal halide bulbs, which cost more to run than fluorescent.

Heres the rundown on my options.

2 x 32 watt bulbs cost about \$10. If I have to replace them every 6 months, that will be 54 bulbs over the lifetime of 1 LED unit (50,000 hours). 54/2 = 27 * \$10 = \$270 for bulbs. 64 watts running for 50,000 hours / 1000 = 3200 kWh * \$.13 = \$416.00 for electric. \$270 + \$416 = \$686 and I havent included the cost of the fixture I would need that can hold 2 48inch T8 bulbs, assuming that would be \$100 or more, so it would be about \$800 to run the fluorescent lights and thats a low-light level.

Finnex LED for \$135.86. 35.6 watts for 50,000 hours /1000 = 1780 kWh * \$.13 = \$231.40 + 135.86 = \$367.26.

The difference is \$432.74 give or take over 13.7 years, or \$31.59 a year. Now I just need the money to get it.