wpg rule on small tanks

[joannde]

I understand that the WPG rule breaks down on smaller tanks. But I don't understand why LOL. Also, at what point (# gallons) is the rule applicable to ?
Are there any WPG calcs that can be made for smaller tanks ?

 

[hashbaz]

The ideal way to measure light intensity is by the power over an area.

Height of water will also limit how much light gets to the plants. Area times height = volume (gallons in our case). Measuring watts per area does not account for how deep your tank is, and this is why we use volume.

But not all tanks have the same surface area to volume ratio.

3wpg on a 75gal is .25watts per square inch. 3wpg on a 10 is only .15watts per square inch.

Someone else will have to tell you the "wpg rule" for smaller tanks, but I would guess that there is no rule of thumb. One might have to find out by experience.

 

[An t-iasg]

Here's a link from one of our members, czcz. He designed a spreadsheet to calculate the amount of lighting you have based on tank dimensions and the type of bulb you have. It's a more accurate way to calculate your lighting levels than the basic watts per gallon method.

http://justanothertank.com/calcs.php

In czcz's link, click also on "new standards" to go to a website from another member, Wizzard~of~Ozz. He also has some tables of lighting information and calcuations. Some very interesting reading!

 

[Purrbox]

The reason that the WPG "rule" breaks down on small tanks is due to several reasons. First and foremost, plants have minimum lighting needs. You have to reach that minimum threshold for them to grow. This means more light over a smaller tank. Another reason is that shorter bulbs usually have lower lumens to watts ratio. Watts is the amount of power consumed, while lumens is the amount of light that it puts out. This means you need more watts to achieve the a similar level of lumens over a small tank.

I've found that Wizard~of~Ozz's lighting page is a fairly good estimate of how much light you'll need. It's not perfect by any means since it doesn't take some of the factors like less lumens in shorter bulbs into account, but it will get to into the ball park. From there you will just need to experiment to figure out what your real lighting level is.

The WPG rule is only truly accurate for tanks between about 50 and 75 gallons. It really starts breaking down badly for tanks under 20 gallons and tanks over 150. In larger tanks you actually need less WPG than the rule would indicate.

 

[Plantbrain]

I just found that small tanks have issues since there is a more limited number of lighting options to cram over them.

I like the light to be even also.
High light is a bad idea for smaller tanks even though folks often do it and suggest it.
Means you have tio tend it more and prune more often. If all you have is 3-4" worth of growing space, stem plants stink!

On a 90 gal, you have well over 1 ft in most cases between trims.

I think the light is more a practical matter, finding the right light to fit over the tank rather than the wpg so much.

Many have gotten on the trend that more light is better, this is not true.
Most all Dutch competition tanks and photos are 1.5-2w/gal of NO T12 type lighting.
These are larger, but the light intensity is going to be less, in deeper tanks, not more.
The angels at which the light hits the leaves is different due to the distance, this seems to play some role IME. More so than the plants needing more light in shallower water, that is counterintuitive.........


Regards,
Tom Barr