Interesting info!
I don't use bead filter, but instead small gravel in my 2 filters (1 or 2 mm). One is 36" and the other is 24". Both with gravel.
I do have settling tank before the 2 two gravel filters.
I normal backflush every 5 or 6 days.
Flow does reduce after the third day. I am still looking for way to filter more stuff out before the 2 tanks but besides this setback, I couldn't complain about anything else.
My water quality is excellent, my koi are all healthy and growing well.
Is it just my luck?
and yes, my fish load is pretty up there.