I learned that the hard way. I am just grateful I had a floor drain for all my learning experiences. Also helped it was all freshwater so not the same kind of $$ at stake!
I found that using the glass crayons my kids had to draw my design on the side of the tank helpful. Then I would sit back with some macroalgae and stare at it and think of the water paths and what happens if stuff clogs.
bad [freshwater] sump but good example of the drawing on the side. It filtered and polished well but was a pain to change the media/clean.
this was a better one: