i dont know really if they make THAT big a difference... but i have a dsb, deeper than it was...theres definately more flow under the rocks than there would have been otherwise.... still getting my flow right though as i have a dead spot under there in one place and its hard to balance without the sandshifting...
i made them after my tank was set up...it would be a lot easier to do it before
my hifin banded goby loves it though as do my shrimp... but its not the be all and end all
i have a differentidea formy new tank build that im working on though...