The best replacement for protein skimmer is refugium.

The best alternatives to protein skimmer are rated as following:
★★★★★  refugium
★★★★★  canister filter
★★★  uv sterilizer
★  surface skimmer
★  power filter
★  algae scrubber
★  filter sock
★  sump
  phosphate reactor
  water change
  purigen

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Speaking about protein skimmer, one probably shouldn't forget about sump.

Other search terms related to protein skimmer and their ratings are:
★★★★★ sump
★★★★★ canister filter
★★★ ozone
★★★ surface skimmer
★★ filter
★★ refugium
★ wet dry filter
★ power filter
live rock
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protein skimmer
refugium
canister filter