Sand Substrate: Red Flint Sand Substrate is naturally occurring in the rivers and streams in nature. This enables familiar environments where sand is needed with your fish species that are not accustomed to fast moving rivers that might include larger stones. Sand also closes the space between particles more so limiting the debris buildup that inevitably leads to breaking down the organics over time. This process may increase nitrates and phosphates and can lower water quality. Red Flint Sand substrate keeps debris on top of the substrate facilitating easy removal and saving time cleaning. Our products are naturally clean – so clean they will require significantly less rinse time.