How it works:
1) Oyster shells are primarily composed of calcium carbonate (CaCO₃). In the slightly acidic environment of aquarium water, calcium carbonate dissolves very slowly. This dissolution process releases carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions into the water, which are the very components that make up "Carbonate Hardness" (KH) or Alkalinity. By slowly dissolving, oyster shell chips provide a steady, passive source of this buffering capacity, helping to prevent pH crashes, especially in smaller or less stable systems.
2) As the shells dissolve, they also release calcium (Ca²⁺) ions into the water. Calcium is an essential element for all calcifying organisms, including corals, coralline algae, clams, and snails. They can also leach a variety of trace elements and minerals found in seawater, such as strontium and magnesium, albeit in uncontrolled amounts.
3) The porous structure of oyster shell chips provides a massive surface area for beneficial nitrifying and denitrifying bacteria to colonize. When used in a filter media bag (e.g., in a sump or canister filter), they can act as an additional biological filter.
Usage: Oyster shell chips are best suited for specific, low-demand scenarios, as low-maintenance beginner tank "set-and-forget" method. Or the contrary, in advanced use in calcium reactors, that is the most effective use in a reef tank. In a calcium reactor, oyster shell chips are placed in a chamber and exposed to water with a low pH (via injected CO₂). This forces rapid dissolution, creating a calcium and alkalinity-rich effluent that is then dripped into the tank. In this controlled application, they are highly effective.
Cautions: Oyster shells are a natural, biological product. Organisms can incorporate phosphates from their living environment into their shells, and then bring those phosphates with those shells to aquarium water. It's recommended to wash the oyster shell chips before introducing to tank, to wash out possible dust, pollutions and their micro particles. Mind the grind size, too fine powder can cloud the water and clog the water pump. When use oyster chips as filter media, it's recommended to use a fine nylon mesh to stuff in first before loading into filter compartment, to avoid tiny chips to run away with running water.