for example tonight i was setting up my sump and the overflow tubes with out taking all the fish out of the 120gal and about half the water which went well. in fact to well. then i went to the store to get sum more media. came back to about 60gals on the carpeted floor due to one tube not flowing right. i went to adjust that one tube that was not flowing right after i refilled the tank up and it broke. that sent the water flowing at about 2000gph down into my sump which has a pump max out put of 1000gph. so i got about 60 more gallons on the floor. so i had to jerryrig a hose to fill the gap till i got back from the home depot with 2 new pvc tubes to replace the bad old ones. so ya.... now things apear to be in working order(knock on wood).
on a side note i decided to go with a pair of discus and tetras.