Bioballs are sphere-shaped frameworks of plastic spars, about the size of a ping-pong ball, which resemble nothing so much as a LEGO tree (the round kind, not the pine-looking ones) without its trunk. They're used in aquarium trickle filters - their large surface area provides plenty of room for the nitrifying bacteria that comprise the first stage nitrogen cycle to grow. They're used less commonly now, as they look interesting and high-tech but aren't nearly so effective as natural filtration.

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