Sure, that’s true, but that’s not what I was seeing and hearing from hobbyists in 2005. When I had a clownfish and a cardinal in a 20g that year, 10 pounds of strategically-placed rock would have been sufficient. However, the rule of 1-2lbs of rock spurred me to get 25lbs. of rock, which made the tank one dead spot and looked awful because there wasn’t much swimming space left for fish that typically swim in the water column.The rock isn't just used as filtation though - it is also used to mimic the natural habitat of, and hiding places for, the fish in the aquarium.