Many manufacturers will recommend a certain amount of sand per gallon of water, but this does not take into account your tank’s dimensions, filtration, or inhabitants. For most systems, you will need at least 1 to 2 inches of substrate. If you have a high flow rate in one area of your tank, your substrate may be slanted and require a gradient between a bare bottom up to 2 inches. If you have digging or bottom-feeding fishes or invertebrates, you may need a slightly deeper substrate to accommodate their building and foraging behaviors. If your substrate is too deep, you run the risk of bacteria losing contact with oxygen in the water. This forms pockets of anaerobic bacteria, which use sulfur as a food source. If they are ever disturbed, they can potentially release deadly hydrogen sulfide into your tank, which can harm your fish and invertebrates. Some advance filter systems harness the filtration capacity of these unique bacteria, but they require constant water flow to be effective in a marine environment, not buried in your substrate.