You're not wrong. Using a UGF
provides both water flow and oxygen, and that allows the nitrifying BB to establish in the gravel. Without a UGF
, the conditions BB need are found primarily in the filter, not the gravel.
This is likely the reason that back in my high school days, fish did not die every time I cleaned the HOB
filter, because we used to toss all the media and add fresh, having no idea about the cycle at the time. But we all used UGFs too, pretty much, and now I finally have a clue why we managed to keep fish alive back then.