I recently added an external bio filter to my pond as an extra filter mechanism. It is set up with a pump in the pond, with a line to the filter and then a line back to the pond. I have small koi, medium goldfish and mosquito fish. I was worried about the mosquito fish getting sucked into the pump so I covered the pump with some netting but the netting gets clogged with algae pretty quickly. The mosquito fish are breeding like crazy so Im not too worried about losing a few of those.
These tetra prefilters screw on most water pumps, you may have to take off the existing cover to get to it. This is what I run when I have tiny koi fry.. https://www.amazon.com/dp/B002RBAT24/ref=cm_sw_r_cp_awdb_5P3Rzb38KXHEQ