My 40 long is 48x13x16 inches. I don’t have a 55 but after a quick search I am seeing numbers like 48x13x20 and 48x12.5x21 I imagine some manufacturers might be ever so slightly different. So the only difference there is about 4 inches of height. But that doesn't matter, his point is that different height tanks (that are the same length and width) will take exactly the same amount of gravel even though they are different gallon-wise, like a 120G 4'x2'x2' and a 150G-XH 4'x2'x31".
IMO the # of gravel per gallon is just as useless as length of fish per gallon or wpg. Different types of substrate weigh differently (although they are sort of close). We are filling a volume so something like liters or gallons and cubic inches would be a better measurement of the substrate (like how Aqua Soil is measured in liters), not pounds. I guess this is one of my peeves lol. Then you would just need to know elementary math, and folks would not have to constantly ask how much gravel they need. I don’t know anyone who wants gravel because they are trying to put a certain amount of weight in there tank, rather than a certain depth.
