Oyster shells are almost pure CaCO3, so they will provide your tank with
calcium, but they don't contain much (if any) magnesium. To supply
magnesium, you have a couple of choices - Epsom Salts are hydrated Magnesium
Sulphate (MgSO4 + 7H2O). Don't worry about adding Sulphate to your tank -
its needed by plants as well (Sulphur is an essential element and is
absorbed as the SO4-- ion). To determine dosing levels for Epsom Salts, use
Chuck Gadd's excellent calculator
(http://www.csd.net/%7Ecgadd/aqua/art...osage_calc.htm)

Another source for both Calcium and Magnesium is Dolomite (Calcium Magnesium
Carbonate - CaMgCO3). You can pick up "food grade" Calcium & Magnesium
Tablets at the drug store in the vitamin and mineral supplement section.
Look for a brand that has no additives or artificial sweetners added
(usually, the generic brands are the best).

James Purchase
Toronto