The V2 Power Box Nitrite & Nitrate Pad absorbs & reduces Nitrites & Nitrates to enhance & improve aquarium water quality for the benefit of the livestock Fits perfectly into the V2 Power Box External Filter Model 200 For best results always ensure good water flow through the filter pad. Replace every 4 weeks, or before, if elevated levels of phosphate are detected in the water The pad should be used in conjunction with regular testing of water parameters