Why does the watts per gallon rule only apply to these bulbs?
So when it comes to bigger tanks, how do you know how much light is actually getting distributed per gallon?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.
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.So when it comes to bigger tanks, how do you know how much light is actually getting distributed per gallon?