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In page Aquarium filter:
"Undergravel filters can be detrimental to the health of aquatic plants.[3] Fine substrates such as sand or peat may clog an undergravel filter.[4] Undergravel filters are not effective if the substrate bed is uneven. In an uneven gravel bed, water will flow only through the thin portions of the bed, leaving the more heavily covered areas to become anoxic.
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