Why doesn't lower wattage mean on more


Guru Class Expert
Sep 6, 2008
between Sac and Reno
I asked this question else where but they just said my idea would not work. So why would lower wattage (42w 10K) mean that I can't run the light a few hours more and have a good result? I am talking say 10-12 hr.
When you guys talk about your lights they are mostly raised above the tank. The post I read said the light spread was better. I found this to be true with my 45g. I then can see some problem I might run into with a raised 42w, reduced light reaching the bottom. I seem to like the look of the 10K over the 6700k I am using now.
Lights are really costly and these are the two lights (sig) I can work with for now. When I raised the light listed below. My tank cleared up. I can continue to use it. It is neat that my visual likes go for the 10k even when the other light is 96w


Prolific Poster
Jan 12, 2009
I'm far from knowing a lot about lighting, but I can say what I've read and what I've understood from my readings.

Plants use light for photosynthesis. How? They use the energy provided by photons to produce chemical reactions.

Less light (intensity) = less energy = less chemical reactions = less "food" = less growth.

What means "less light"? Less light can be less bulbs, lower wattages, or (until some point) less light time.

Then, if you want more growing, you must raise your plants' energy (light). You can add more wattage, lower your lamps (put them more near the surface) or provide mor time of lighting.

The two first ones are just limited by the amount of nutrients (mainly CO2) your water have, or the hability to provide the light under the surface.

The third one has another limit. Your plants cannot be "photosynthesizing" the whole day. Every plant has a time limit after it ceases its photosynthesis and rests for the rest of the "day".

Experts say this limit is 12h.

Then, if you are providing the maximum amount of light intensity you can to your plants, but you have not reached yet the 12 hours limit, you can, as far as I understood what I wrote, give more time of light, and you will have a small benefit. But once you cross that 12h line, you will just annoy your critters that will kill their mother to find a dark place to sleep.

About light "temperature", VaughnH posted somewhere around here some conclusions about. He says there is no real difference between the growth of plants under different temperatures (and the same wattage/intensity). It means you can just put the bulbs that more please your viewing.
But it means too you can follow the "traditional" voices, and put merged bulbs without risking yourself too :D


Guru Class Expert
Feb 17, 2007
Lincoln, UK
I'll use wattage as a 'marker' as it is easier for most to understand wattages than other measurements.

There is no limit to the hours you can put light above the tank. 24 hours is not a problem for the plants, however there isn't much point because once they've had enough they will 'shut' their leaves and take no more. You then risk supplying light for algae to grow if there is the slightest defficiency. Also as said above let the plants and fish have their rest.

12hours light/12 hours dark is what they will most likely get in nature. We tend to go for shorter periods as all we want to do is give the plants enough light for their growth and minimise the algae's chances, therefore the norm in planted is 8-10hours. Often we will just use 5-6 hours when we first plant and gradually increase it each week.

If you have 2 lights in a hood and to your eyes it looks bright on the substrate then when you raise the light it may look darker. Not necessarily less light reaching the substrate though. More likely that it is now covering the whole substrate better rather than being intense in one area and not too good in others.

You prefering the 10K over the 6700K is more than likely you prefer the colouration. It may seem darker or brighter but it will be as 'bright' to the plants if they are the same W.

By the way lights are not costly. You already have the equipment. the tubes are only costly if you pay for the following 8 letters to be printed on them: A Q U A R I U M. I think they use a very rare and expensive ink to print this word ;) Get a decent normal tube that fits your unit (wattage and length) from a normal hardware store. It is the same bulb without that expensive printing on it.

As an example in the UK a standard 18W T8 'non aquarium' tube will cost under £3. A decent 18W T8 'non aquarium' tube will be circa £5. The Arcadia standard AQUARIUM version £11, Arcadia Plant verison £15 and ADA NA £20.

The standard under £3 will have a poorer CRI (maybe 765/865) meaning it is not as good quality (not as accurate in colour rendition etc) therefore we would buy the 'circa' £5 one which would be a 965. Same wattage, same output. Marketing sells the aquarium ones and some of the aquarium ones may not be much better than the 765/865 cheapy household one!!! They rarely tell you the actual CRI on the aquarium ones. Just loads of pictures of spectral graphs.

My flourescents weren't raised above the hood!! They were 4-6" above the water. My LEDs are raised even though they are lower wattage because they are much much brighter W for W. However being brighter doesn't mean the plants are getting more light. It means your human eye is seeing more light, but I can see from plant growth that the plants are also getting more light!!!

I wouldn't suggest that lowering lights will raise the energy that much. It will for the plants directly under it but others further from this point may get less!!! Depends whether how many tubes you are using and how they are spaced/positioned.

As for colour temperature. If I lower my LED light close to the surface I can see rippling on the wood cabinet directly in front of the glass. Guess what. The light seperates out and I can see red/blue/green in this rippling. These are 5500K LEDs and from the rippling I can see the 'full spectrum'.

I have grown plants under pure 7500K, 6500K, 4000K, 4500K, 3000K and have never noticed any difference in growth speed. I have noticed differences in colour but turn the lights off and the colours are all the same. It is the bulb colour that gives red a deeper red or a nearer peach etc.

Summary:Run the light for 12 hours, 14 hours, 16 hours. If you have the desired result then you have your answer. There is never any harm in trying things like this because if it doesn't work you just go back to what did work (reset and try something else.)

This is a tank with a white background.

This pic shows 0.9WPG of 4500K. The tank has a pink hue:

This pic shows 0.9WPG of 6500K. The tank has a green hue:

This pic has 1.8WPG. the 2 above combined. The tank is white:

These are T5HO tubes though. Do the colours look the same to you? Does the white look white? The above is my eyes perception of the colour. Your eyes may seem different. Look at Monet's paintings as he got older. The colours turned more and more yellow because that what his eyes saw as he aged!! Not all of us will see the same just because we are not old. Each person's eyesight and perception can differ dramtically.

What do you see in the above 3 pictures?