Partial sunlight tank

Hmm...let's see...
...your lights, 2X20W 6500K = 40watts of light vs.
...THE SUN, 999,999,999,999,999,etc....Watts of full spectrum light :idea:

Pitting the sun against any lighting is like pitting lightning against a watch battery...more or less

-OR-

Think of it this way...
What happens when you touch your light bulb? What happens when (if you could) touch the sun? A little discomfort in your fingers vs. you melting :devil:

O.k., I am just being silly but the point is, the sun produces way more energy than any bulb or bulbs.

40w isn't alot of light energy, which is one of the reason the wpg rule doesn't apply correctly to small tanks. The wpg applies well for tanks within the 20-100g size and common shape but it has it's flaws. If you try the wpg rule on small tanks you aren't giving the plants enough energy to work with and the opposite is true at the other end, where even 3wpg over a 200g tank is too much.

Well, you might compare lumens per square feet. Your 40 W, with roughly 70 lumens per W, equal about 2800 lumens, or about 2000 lumens/sq.ft. for your 10 gallon, or about 1500 lumens/sq.ft. if you allow for losses from a non-ideal reflector. That's more than an overcast day delivers (1000 lumen/sq.ft.), but less than a sunny day (10,000 lumens/sq.ft.).

Keep in mind that the value for the tubes is only valid directly below the tubes.

Less light on the 10 then the larger 50gal. This seems backwords to me could you please explain this a little clearer as I am confused and would love to see the logic. If I am wrong and you are right then I have a major upgrade in my knowledge. If I am right and you had those reversed then (NENER NENER NENER) then you just brought up a subject that I would still be interested in learning a little more about and would love you to write more (after clearing your mistake) as you seem to know this subject much better then I.

I can only think of the fact that, given my example of the 10 gallon tank with 4 W/gallon above, a 55 gallon tank with 4 W/gallon would have 2700 lumens/sq.ft. in comparison to the 1500 lumens/sq.ft. of the 10 gallon tank. In principle, this evens out at the substrate level, because the 55 gallon is deeper, but in reality, this seems to play no role.

jennfier said:

On sunny days, my 10g is getting about 1 hour of sunlight at an angle near sunset. When that happens, it seems that everything is pearling. It's great for the smaller plants in front that are somewhat shaded from aquarium light by the taller plants. I've had this set up for a few months now. My timer is set to be off when the sun hits and counting the sunlight hours, photo period is about 7-8 hrs. I had algae in the beginning but since scaling back the hours, it has almost disappeared.

My question is, why are they pearling more to the sunlight than my 2x20w 6500k bulbs that are only a few months old (a little over 2 months) ? A couple of the plants pearl no matter what but all the others only pearl for the almighty sun (and of course after WC).

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I have a tank thet gets filtered sunlight and supplemented by only 20 watts on a 29 gallon tank..

the plants are growing like crazy..I put elodea in it ..I started with 4 fronds..I now have a back wall full of it..I have to trim it every week then replant it..

I just added CO2 to this to see what happens..

jennfier said:

Sorry if I'm a little dense. Wouldn't the light source being closer to the plants in a small tank be giving the plants too much light instead ? I'm equating the energy that you mentioned w/ light. Can you break it down for me mathematically or does it have something to do w/ diminishing returns ?

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Even if the light is much closer, you're still dealing with just 40w of light compared 200w in a 50g tank. Depth does play a roll to an extent but difference between the height of a 10g tank and that of a 50g tank isn't that much, about a ft at most. Some energy will be lost after the light has traveled that foot but not enough to reduce the impact the significantly higher amount of light.

The wpg rule isn't a hard set rule, it's a very general observation made to simplify the lighting issue. It works rather well for tanks within a given parameter (20g-100g tanks using NO fluorescent lighting,standard tank size, etc) but it does not cover all parameters. People have tried finding a better rule of thumb, like using watts or lumens per water surface area instead of volume, but that can get overly complex or just flat out wrong.