Undergravel filter: (n.) Undergravel filters (abbreviated as UGF) are mainly biological filters. The filter fits onto the very bottom of the aquarium and is covered with substrate. Airstones or powerheads pull water down through the gravel bed and promote beneficial bacteria growth in the substrate. It is important to use a gravel vacuum to remove debris from beneath the plates during cleaning. If debris builds up, ammonia levels will increase.
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