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SESK: Stuff Everyone Should Know · sesk.info/fish-undergravel-filter

In an under-gravel filter, what acts as the filter?

The gravel itself, which traps dirt and holds helpful bacteria.

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  1. A paper cartridge
  2. The glass walls
  3. The gravel itself ✔

Why: A grate sits under the gravel and water is pulled down through it. The gravel traps particles and grows the bacteria that remove ammonia.

Huh, didn't know that: Pile the gravel a little higher at the back, near the lift tubes, so it filters well.

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Why don't fish play piano? You can't tune a fish.
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References

Answer: The gravel itself. A grate sits under the gravel and water is pulled down through it. The gravel traps particles and grows the bacteria that remove ammonia.

  1. Merck Veterinary Manual (pet owner): Providing a Home for Fish www.merckvetmanual.com/all-other-pets/fish/providing-a-home-for-fish
    In an under-gravel system, the gravel acts as the filter medium, which traps particulate matter and contains nitrifying bacteria
    Checked 2026-10-11.

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