If you need to use buckets for filling any tanks make sure they work with the stand ..
That is sort of how I try to figure space from top of one tank to bottom of other besides taking the limits into consideration.
If you are going wood instead of blocks this is my favorite rack that stacks tanks on top of each other..

By notching "dado' the 2x4 that runs front and back is held up by the 2x6 not fatseners ,they just keep things in place..
To make them look nicer I add 1x5 to the front runners to cover tank bands but they are not needed.