i've read lots of discussions about filtration and am working towards my own conclusions, based on the results of my "tests"
it's my understanding that your growbed is your biofilter, some argue that additional solids filtration is required, but i've seen documentation of mature systems in 3rd and 4th years of productions without cleaning, by utilizing worms in the gb's..
additional filtration (there's 2 main types, mechanical and biological), imho, both would be required if you are pushing the "fish" load on your system - my thinking is based on "rules of thumb" that are argued for/against regularly..
i'm gradually "growing" a very humble system compared to most of the "backyard" systems you see here, but it has sunfish,(some kind of bluegill) yellow perch, rosy red minnows, daphnia and scuds, i guess because i don't have enough to think about! lol