It seems to be generally accepted that small tanks need more watts per gallon than large tanks. This seems counterintuitive to me, because big tanks tend to be deeper than small ones, and hence more energy would be required for the light to reach the bottom, but I expect that my intuition is wrong.

Why do small tanks need more WPG than large ones?

Is this true for low light tanks as well as high light tanks?

Thanks.

Bill

It's a rule of thumb thing that doesn't work well when talking about very large or very small tanks. I can't say I know the orgin, but it does work for tanks from 10g on up to what, 200 or 300 gallons? Maybe more?

On the small end, it can get perty ridicules. I have a 3 gallon Eclipse on my desk. It has a 6 watt T5 light. Six watts of light isn't very much at all for growing plants. The rule of thumb says I've got two watts per gallon and should see some impressive plant growth. I consider it a low light tank and see hardly any growth at all. The Anubia has sprouted two leaves in 5 months. The Java fern is about the same size. The Ludwigia is thriving, but it has very slender leaves compared to the same plant in my large tank at home.

If you had a quart jar full of water, could you grow high light plants in it with a 1 watt light? After all, that would be 4 watts per gallon!!

steve

I've always done well, even with odd shapes with the old w/gal rule.

I think I've done enough tanks and custom work to say that is an awfully good rule.

But I do have more w/gal on smaller tanks, it's main due to shaing issues from other plants and the fact that the light does not spread out enough and finding a 2 to 4 watt light is tough!!!

Basically spread of the light and practical sizing issues come into play.
But over all I use 4-6w/gal for smaller tanks, even some non CO2.

Larger tanks seem to do quite well with 2 w/gal for the reverse reasons, lots of spread, easy to add more light.

More light is seldom ever better.

Few have tanks have depths more than 24" and I have not found that they need more light at that depth. I've done 36" deep tanks, those suck, not because they have light issue, but due to one's arm length!

Regards,
Tom Barr

I found this to be the case with the light unit I had not covering both front to back and side to side in my 20g. With one light on for 2X55watt pc unit plants were living not thriving. Turn second light on, spread of light is excellent and plants are growing... well like weeds ;-p




I can definitely vouche for this, my 75 gallon tall glass is still collecting dust due to how much of a pain it was to clean out. This tank if I recall was 36" tall and completely impossible, short of arm extensions, to work on anything in the substrate.


Ken