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What are they?
These are usually blue or black plastic, virtually hollow balls of various sizes (2.5-5.1cm (1-2") in diameter which are used in the biological nitrifying filtering of large external filters.
Their shape ensures complex water paths through them which keeps the bacteria on them supplied with oxygenated aquarium or pool water which the bacteria feed upon extracting any ammonia and nitrite.
- Alternative objects exist with much higher surface area exist like sintered glass media (Eheim Substrat Pro or Bio-rings for example), Alfagrog or Seachem Matrix media.