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It doesn't apply to any bulbs. It is a coincidence that 2 watts per gallon of T12 bulbs, either 24 inch or 48 inch bulbs, will give enough light to grow some aquatic plants. Watts spent powering a light bulb are not the same as a fertilizer, which you can mix in the water. A given amount of fertilizer gives a fixed ppm per gallon of water. But, a given amount of watts doesn't mix with water, so the number of gallons of water you use it for is irrelevant. You get about the same light intensity if the tank is half full of water as you do with a full tank of water. Chop off the top half of the tank, holding the light fixture the same distance from the bottom of the tank, and you still get about the same light intensity. Gallons has nothing to do with light intensity.
 

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Discussion Starter · #3 ·
It doesn't apply to any bulbs. It is a coincidence that 2 watts per gallon of T12 bulbs, either 24 inch or 48 inch bulbs, will give enough light to grow some aquatic plants. Watts spent powering a light bulb are not the same as a fertilizer, which you can mix in the water. A given amount of fertilizer gives a fixed ppm per gallon of water. But, a given amount of watts doesn't mix with water, so the number of gallons of water you use it for is irrelevant. You get about the same light intensity if the tank is half full of water as you do with a full tank of water. Chop off the top half of the tank, holding the light fixture the same distance from the bottom of the tank, and you still get about the same light intensity. Gallons has nothing to do with light intensity.
So when it comes to bigger tanks, how do you know how much light is actually getting distributed per gallon?
 

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So when it comes to bigger tanks, how do you know how much light is actually getting distributed per gallon?
You don't distribute light per gallon. Instead, you light up an area by having a light a specific distance from that area. If you want lots of light you use multiple bulbs close together, or you use brighter bulbs, like T5HO instead of T12. To get good uniformity of that light you try to match the bulb length to the tank length, and space bulbs across the top of the tank so they aren't all bunched in the middle.

You can limit your choices to T5HO and PC (like AH Supply kits), and it isn't hard to figure how many bulbs you need, at what height above the substrate to get high, medium or low light intensity. If you can get a PAR meter on loan, you can adjust the height of any light fixture to get the type of lighting you want.
 

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Discussion Starter · #5 ·
I have this over my 10 gallon, and I've never been able to keep red plants. It's only about 2 inches above the top of the tank.

Would I either have to up the wattage or add more light some other way to accomplish this?

If I add more light, would I have to up my CO2 to prevent algae, and put my drop checker toward yellow instead of its current green color?
 

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That seems to be a 13 inch long bulb, but the tank is 20 inches long, I think. So, the bulb should be good for the middle of the tank, but the ends will not be well lighted at all. And, it would be best if that bulb had a color temperature between about 5000K and 10,000K, not the actinic one shown on that website. I don't think that fixture has a reflector other than the metal shell of the fixture, so it won't give nearly as much light as it would with a good reflector.

I don't think you need more light to get the red colored plants to look good. You do need a light that has some red in its spectra, which you may not have. Without red in the light's spectra no red can be reflected for you to see as a red plant. Also, be sure you are providing nitrates, phosphates, potassium and trace elements along with the CO2.
 
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