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This is further evidence against the Watts per Gallon rule, which was always just a rough guide. In other words, 40 watts over a 10 gallon tank may seem like a lot of light (4WPG), but in fact, it should have closer to 65 watts. The reason for this is due to surface area, but exactly why I haven't quite gotten a handle on yet, so I'll do more research and report back when I have a better idea. All I know is that smaller tanks demand more light per volume and larger tanks demand less. The WPG rule holds for the most part in medium sized tanks (about 29 gallons to 55 gallons), but below and above that, it's better to go by something like this tool, or do your own calculations using surface area (-gulp-). Personally, I like the tool...