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How much light is enough light?


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I have a 100 litre tank with a 11W LED light. Does this light have enough lumens to grow “advanced plants”? According to tropica, around 1 Watt per litre is needed for plants like these. 
Also on average how long should the lights be left on for to ensure best plant growth? 

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A good light, like the Aquarium Co Op or Fluval Planted 3.0 might be in the 30+ watt range for that size tank.  Those are lights that have good spectrums for plant growth and, I think, at the 30 watt level would grow low to medium light plants.

I don't think a generic 11W LED light (not sure of the brand) is going to be able to grown "advanced" plants on a 100L tank.


Edited by Galabar
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I wish it was a simple question... 

What plants are you growing (ie what is their light demand)? What is your expectation for growth rate? [These are rhetorical, but share the info if you want]

I've found that cheap lights will grow most of the more common or unfussy plants, slowly, in a fairly shallow tank. But with slow growth comes other problems, like algae. So probably the light you have will will work, if your plants aren't too fussy and your expectations aren't too high. 

More expensive lights (this includes mid-range prices and high end, bc the line between them in terms of performance isn't as great as it is bw low end and mid-high end) will accelerate growth, and support more demanding plants. You may need ferts and/or CO2 to see these improvements. These lights will also make a big difference for carpet plants. 

In terms of what to look for on the package/details of your light, look for info about lumens or lux. Unfortunately, just looking at that info on its own is less useful in terms of "will this light work for me?", unless you are using it as a point of reference or comparison. Eg I have light x over my tank, and it puts out ~2000 lumens. I find that really bright, so I could probably manage with a light at 1000-1500, then adjust height etc to dial it in. That's why dimmable lights are AWESOME; you shop for something that  greatly exceeds your demand, then dial it back to what works for your needs and setup. The fluval 3.0 and coop light (from what I gather) fall into this category, ie few casual (low-tech) users will use them at full strength. 

Just trying to share the thought/shopping process.

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It’s not so much about how long they take to grow but that they grow properly. I have eleocharis in my tank which I want to carpet however I have noticed it’s growing up rather than out (it doesn’t seem to be carpeting but it is healthy - it’s green in colour and not turning brown) 

I’ve also got elodea in the tank and I would like to add Alternathea, but I would want to ensure I have the right amount of light to make sure it grows properly. 

I wouldn’t want to spend hundreds on a light, I am unsure if there’s one in the £60ish region that works well enough. 


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On 5/16/2023 at 1:10 AM, Izzyf said:

100 litre tank

what is the tank height?

On a 10-20G tank (short height) I have my low power light at about 20-35%
On a slightly taller tank (29G) I had to push the 3.0 light up to about double the strength to hit carpeting plants.  Using a riser actually might help with that too because it spreads out the hotspot to the whole base of the tank.
On my bigger tank, 75G, it is slightly taller again and I have that 3.0 at 75% or so and raised up 8".

Edited by nabokovfan87
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