Koi Pond Filter Questions Koi

I recently got a free hard plastic Koi Pond Liner. I have had freshwater aquariums for several years but have never dabbled into Koi or Goldfish Ponds. I added a river gravel substrate to the bottom of the pond to hopefully help with plants as well as the growth of good bacteria. My question is what type of filter should I get for this small pond? I believe the filer needs to submersible as i dont have the capability for a true waterfall filter.

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