It's a very rough metric. In one sense it'd be easier to convert the tank
to gallons then use the existing rule.
You're in Canada and so am I and while this is 100% a metric country,
aquariums are still sold in gallon sizes.
Watts per foot or square meter might work for you, but as Tom points
out depth needs to be taken into consideration.
But, it's a rough metric and has always remained a riough metric because
it's not that important. After a while you'll know what you're doing more
and have an innate feel for it.
What size is the tank and what lights do you have on it now??
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