The best replacement for purigen is chemi pure.

The best alternatives to purigen are rated as following:
★★★★★  chemi pure
★★★  activated carbon
★★★  hypersorb
★★★  gfo
★★★  phosguard
★★  chemi pure elite
★★  skimmer
★★  bio chem zorb
★★  nitra-zorb
★  protein skimmer
★  chemipure
★  chemi-pure elite
★  seagel
★  poly filter
  phosban
  chaeto
  macropore
  renew
  chemi-pure

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Speaking about purigen, one probably shouldn't forget about carbon.

Other search terms related to purigen and their ratings are:
★★★★★ carbon
★★★★ stress coat
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purigen
chemi pure
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