I have found the foam fractionation works well with fish that consume a good amount of protein.
On the pond below there is a similar pond filter to the one in your pic, along with a cascading DIY protein skimmer.
The fraction process removes excess protein as foam that drops on the ground.
I have also used them on carnivorous tanks, where water protein is collected in all sorts of
needless to say, waste had to be emptied at minimum daily