Is the 55 not 18 high and the 90 twenty four inches high ewok? You are also talking about the preasure being applied to a much larger area. In the airstone case the preasure is applied to a very small space and is the same in any size tank I think. I am not 100% sure here and I may be wrong but water preasure is as you say a function of weight which is in itself a function of gravity which is a downward preasure. Perhaps someone with more knowledge in science and such could pipe in and set things straight.