It's just a generalized rule of thumb kind of like the 1'' of fish per gallon rule, although that hardly applies to anything anymore. It's basically from what I can understand a way to ensure you have enough light penetration according to the size of tank. Say, while two of these bulbs would be enough to grow plants in a 55G they surely wouldn't be enough for decently larger 75G. So 20-30watts may be plenty enough for a 10G, it would hardly grow the easiest of plants in a 30G.
I dunno if this helps, it might not make any sense at all haha. All in all though it's not about containing the light, it's just about making sure you would have enough depending on the size tank.
One thing I've been wondering though is if WPG IS everything and how big of a role lumens play. I recently switched from T12 to T8 bulbs because I wasn't seeing much growth with the T12s now for me it went as follows
So, from what I gather is eventhough the T8 is lower wattage it produces more light, so would that mean better plant growth?
Also, it seems that the smaller a tank gets the less the WPG rule applies. Surely in a 1G tank you wouldn't be able to grow glosso with 3 watts total of light. I just last night put a 20watt bulb on my 5.5G but, Im not sure if even that will be enough to grow plants, guess I'll just have to see.
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