Another potential issue in the breakdown of the WPG rule for small tanks is the fact that smaller bulbs produce less light for a given number of watts.
Take for example a 48" 32 watt T8 bulb. A good one should produce 2800 lumens or 87.5 lumens/watt. Now, the same tube in 18" 15 watt T8 form should produce 1312.5 lumens. However, I have never run into an 18" bulb that produces this much light (If you do let me know!). The most I have ever seen is between 700 - 800 lumens (Hagens LifeGlo bulb might be higher as that is a pretty bright bulb!). This means that 18" 15 watt T8 actually produce around 47 - 53 lumens/watt. This is even lower than a 48" 40 watt T12, which is around 59 lumens /watt.
This may explain why some of the best small tanks has an insane amount of lighting...even if you take into account surface area. Using this kind of calculation (and surface area) a 10 gallon tank with 4 18" 15 watt T8 bulbs will still effectively only be ~2.5 - ~2.8 WPG.
Am I totally off base here?
Take for example a 48" 32 watt T8 bulb. A good one should produce 2800 lumens or 87.5 lumens/watt. Now, the same tube in 18" 15 watt T8 form should produce 1312.5 lumens. However, I have never run into an 18" bulb that produces this much light (If you do let me know!). The most I have ever seen is between 700 - 800 lumens (Hagens LifeGlo bulb might be higher as that is a pretty bright bulb!). This means that 18" 15 watt T8 actually produce around 47 - 53 lumens/watt. This is even lower than a 48" 40 watt T12, which is around 59 lumens /watt.
This may explain why some of the best small tanks has an insane amount of lighting...even if you take into account surface area. Using this kind of calculation (and surface area) a 10 gallon tank with 4 18" 15 watt T8 bulbs will still effectively only be ~2.5 - ~2.8 WPG.
Am I totally off base here?