Most aquaria are 1x1x2 proportions. Put the gallonage up by a
factor of 8, the area to be lit goes up by a factor of 4 and
the distance to the bottom by a factor of two. Although the
light intensity falls off as the square of distance for a point
source, it falls off only as the distance for a line source
such as we use in aquaria. So, 8 times the gallonage does
match 8 times the light required.

For deep tanks (tall format), light does not fall off as much
as you would expect from the line-source rule compared to
1x1x2 format: the angle at which a lot of the light hits the
glass-air boundary on the sides at less than the critical angle,
so is reflected back into the tank.

Watts/gallon is a surprisingly consistent measure to use. My only
reason for enquiring about lamps used is to convert it to units
available to the 90%+ of the world that has access neither to
US gallons nor to US lamps.