Big, yellow water lettuce under LED/ small, green water lettuce under CFL. Why?

[morainy]

Hi, Tom

One of my sons has been doing a project for his science class, comparing LED and fluorescent nano aquarium lighting. One of his experiments has 2 Fluval Specs with identical substrate, water, plants, etc. Plants in both nano tanks are growing surprisingly well.

The water lettuce under the Fluval Spec LED has grown very large but is on the yellow side.

The water lettuce that has the Azoo fluorescent light is smaller and the leaves are darker green.

The water in each tank is the same because every day we empty half of it and add water from an established planted aquarium.

Can you think of why the water lettuce under the LED would be yellow and bigger? Could it be the effect of the LED light? Or could it be that it's grown faster and therefore depleted the water of nutrients more quickly? There are a lot of plants in these Specs, so they could be using the nutrients up quickly even though he replenishes.

He got the technical information about the Fluval LED from Hagen. I don't have it here but it's got 31 tiny LEDs. The information that Fluval sent my son is very hard to read (bad photocopy) but it looks like the luminous flux is 125.11

Thank you,
Maureen

 

[Tom Barr]

Light difference alone could be the cause, in both cases, I'd add more KNo3 and maybe some GH booster.

You do not need much and recovery should not take long, maybe a week or two.

 

[morainy]

Hi Tom,
Thanks for your reply so late at night. My son isn't worried about improving the look of the plants because it's a science project. He's wondering whether it's the brightness of the lights (lumens?) or the kelvins that are affecting the plant growth, or if it's something else.

His other tank, a 22 gallon long tank with a CFL at one end and an Eheim Aquastyle 1200 lumen LED at the other end, is showing that elodea grows quite thin and sparse under the LED, but much thicker and faster under the fluorescent; even a piece that's rooted under the LED will become thick if it can drift across the water and be partially under the fluorescent. The other plants are mostly doing the same under both lights, except for limnophila, which loses its needles mid-stem under LED.

These are in the budget light categories and can't be compared to expensive LED lights made for aquarium plant growth. It's just a class project, not a full science fair project.

 

[Tom Barr]

Likely the intensity of light is much stronger in one casr than the other.

Starting and present total plant biomass might be okay for one tank, and if he added more to the other, the plants might be eating themselves out of house and home.
So are the biomasses the same in both tanks?


Lumens are poor measure, but looks like all you have. Cameras can sort of measure the light also, they use lumens.
It's weighted around 555nm, PAR for plants is fairly weighted over the entire used spectrum: 400 to 700 nm.

 

[morainy]

Thank you, Tom

Interesting question. For the 22 gallon long with the lights at each end, the biomass is the same because the plants and lights are all in one tank, and the same plants, of the same size, went into the tank at each end at the start of the experiment.

For the 2 Fluval Specs, which are 2 gallons, the biomass started out the same as all the plants were measured and cut before planting. But it's not the same any longer. It looks like under the fluorescent light, there's at least double what's under the LED now.

Thanks for the tip. I'll give your email to my son.