Rex's Planted Tank Guide

Your Source Of Information For Planted Tank Aquariums, Lighting, CO2 & Fertilizers

Minimum Light Threshold

Why your 10 gallon tank needs more light than the WPG rule states.

Take a one gallon tank. Put a 4 watt bulb on it. You now have a 4 watt per gallon tank. But wait! It seems that nothing will survive in your “high” light micro tank. Why is that? Well plants need a minimum threshold of light to survive and grow. And this minimum is not expressed in the watt per gallon rule.
Here's the skinny on light energy. At noon in the tropics where a lot of our plants come from the sun provides up to 140,000 lumens per square meter. If you take your 10 gallon tank and look at the surface area it has you would have to have almost 220 watts of light on it to equal what the sun provides. Kind of makes your 20-40 watts look insignificant doesn't it?
Now if you did somehow manage to cram that much light over a 10 gallon tank you would have to devise some kind of cooling system and a system to automatically keep the tank filled. Because just like in the tropics that water is going to be doing some heavy duty evaporation. But it does point out why the watt per gallon rule breaks down.

If you take a standard 55 gallon tank you would have to have 660 watts of light over it to equal what you get in the tropics. Now, let's take a more reasonable figure of lux on a square meter say 75,000 lux. Using this figure we come up with 117 watts over the 10 gallon tank and 353 watts over the 55 gallon tank. Now I know some people who are using over 300 watts on a 55 gallon tank and getting some great plant growth. I also know one person who has used 72 watts over a 10 gallon tank and gotten great plant growth. But these very high light tanks are hard to keep balanced.

So let's use a known example. Let's take a 55 gallon tank with 220 watts of light. I think that everyone will pretty much agree that you can grow anything you want in this tank. This tank has right about 30 lux per square inch of surface area. If we take that number and apply it to a 10 gallon tank we end up with a recommended high light value of 6000 lux or 71 watts!!!!! Note all these lux values for light are taken from the 6700k bulb sold by AH Supply.

So to make it short you would have to have 71 watts of light over a 10 gallon tank to get the same effect you would have with 220 watts over a 55 gallon tank. Now I know the 10 gallon tank is almost half the height of the 55 so you will be able to get by with a bit less light than that. But it does I think show why the watt per gallon rule really doesn't work that well on smaller tanks. And to prove this point let's look at a larger tank where we know the watt per gallon rule also breaks down

Our example here is going to be a 125 gallon tank. It's pretty much well known that if you hit 3 wpg on a tank this size you can grow anything. So what kind of lux value does that give us? 24 lux per square inch. Compare that with the 55 gallon tank which has right about 30 lux per square inch. Pretty close.


Lux


So what I am going to propose is that instead of thinking in watts per gallon we instead think in lux per square inch or LSI. Let's look at these figures based on a 55 gallon tank with 110 watts of light (low light), 160 watts (medium light) and 220 watts (high light).

  • Low light is 15 LSI — you can grow a fair amount of aquarium plants.
  • Medium light is 22.4 LSI — you can grow most of the aquarium plants available.
  • High light is 30 LSI — you can grow all aquarium plants.

And since those are hard numbers let's instead make them ranges.

  • Low light is 12-17 LSI — you can grow a fair amount of aquarium plants.
  • Medium light is 20-25 LSI — you can grow most of the aquarium plants available.
  • High light is 28-32 LSI — you can grow all aquarium plants.
  • Very high light is over 35 LSI — same as high — all doubts removed.

Now I know what you are thinking. I can do watts per gallon pretty easy. But how do I figure this LSI thing? Well find the surface area of your tank in square inches. Then take the LSI number you want to reach. Multiply the surface area by the LSI. Take the result and divide it by the LUX rating of your chosen light. I will admit that this system is not perfect by any means. But I think it gives a better idea than the WPG rule