Watts per gallon for low, medium, and high?

[gwlee7]

I keep hearing everyone talking about low, medium, and high light plants which I understand quite well since different plants have different light requirements. What I don't understand (or haven't run across yet) is a rough guideline that helps with watts per gallon as it relates to low, medium, and high light plants. For example, both of my tanks are lit at 2 watts per galllon. Where does that fall? Are they low light or medium?

Thanks.

 

[blue_ram]

This is a rough guideline for determining high/med/low lights

Very Low: < .5 watts
Low : .5 to 2 watts
Medium: 2 watts to 3 watts
High: > 3 watts

This is guideline is valid for aquariums of 18" in height, and is at least 20 gallons. This assumes that you have no glass/acrylic shield over the lamps, and water is crystal clear with no surface film. Otherwise, you'll have to up the wattage (don't know a rule for that though). If your aquarium is shorter (e.g. 20g long), then you can get away with slightly less wattage. OTOH, if your aquarium is taller, then you'll have to increase the wattage.

While back, a friend of mine did an interesting experiment. He used a professional light metering device used for photography to measure the intensity of the light in his 75g (or was it 90g?) aquarium. The intensity at the surface of the lamp was more than 4 times the intensity at the bottom of the aquarium.

 

[enchanted]

blue_ram is sort of right, except that it doesn't change as much on hieght but on size (gallons) of the tank.

This link explains the minimum light threshold:
http/www.rexgrigg.com/mlt.htm

Basically the smaller the tank, the more light, the larger the thank, the less light, needed to achieve each threshold.

There are also multiple areas:
Low = Less than 1WPG
Low-Medium = less than 1.5WPG higher than 1 WPG
Medium = Higher than 1.5WPG but less than 2WPG
Medium-High = Higher than 2WPG, but lower than 2.5WPG
High = 2.5WPG - 3WPG
Very-High = 3WPG

This is for mid-sized tanks and does not apply to tanks under 20 Gallons nor tanks over 55 Gallons.

If you read the link you will understand why.

 

[Lateral Line]

Watts per gallon is frequently quoted, I do so myself because it is what people relate to, but the relationship is very complicated. It is more reasonable to quote Watts per surface area/depth. People baulk at the prospect, so you get answers like 2 Watts per gallon is "Medium".

For regular shaped tanks of common sizes, the figures above work well enough.

 

[gwlee7]

Thanks for the replies. I have been reading the discussions re: lighting for plants and it does seem to get complicated. I don't want to get plants that will die in the amount of light that I have but I'm not quite ready to go the compact flourescent route yet either.

To help the discussion:

My 55 gallon is the standard 18" tall and the 30 long tank is 16 1/2" tall. They have 110 watts and 60 watts of light respectively so each has "2 watts" per gallon.

I realize that it is more "complicted" than that but am just trying to get a handle on what types of plants I'll be successful growing.

 

[NickT]

i found 80 watts kept anything i put in my tank alive even dwarf hair grass witch i was told wouldn't last in that much light. I'm upping to 260 watts Wednesday so i guess I'm going to see a difference in the amount of growth..... i think even the 40 watts of light my tank came with wold keep some plants alive but its growth everyone wants......

 

[enchanted]

DJ, how long have you had your tank? Sometimes lighting deficiency can take time to show, but eventually the plants will die without proper lighting.

Listen to experience and research it. If you research you will find that people constantly find the problems they are having is due to lack of lighting and failure to research the lighting requirements of plants.

There are low light and medium-low light plants that can do well in low lighting and will grow slowly, but medium to high plants will eventually dwindle and die off.