Hardness is primarily the concentration of calcium (Ca++) and magnesium (Mg++). the two most important metal cations with more than a single positive charge, but in aquaium usage it sometimes also refers to alkalinity. Real alkalinity is the acid neutralizing capacity of all the negatively charged ions, but mostly phosphate (PO4-), carbonate, bicarbonate, silicate, and borate. Aquarium alkalinity and hardness can also be expressed as Mg CaCO3/L milliequivalents per liter (meq/L.) Aquarium hardness is usually reported as mg/L of calcium carbonate, but in reality hardness should refer only to the calcium concentration. When we talk about millequivalents, we are referring to the amount of base or alkali (OH-) it will take to neutralize the concentration of anything with a positive or acidic charge (H+) or more than one charge if it is a metal rather than an acid (Mg++).\r\n\r\nIt is important to raise aquarium concentrations of calcium to that of seawater and alkalinity even higher for three reasons: 1) Calcium, depleted by stony corals and coraline algae, must be replaced. 2) Alkalinity helps maintain a high ph. 3) Calcium saturation will precipitate phosphates, protecting the aquarium against nuisance algae blooms. (Calcium carbonate is less soluble than calcium phosphate.).\r\n\r\nShould you use baking soda (sodium carbonate) to adjust alkalinity? Not recommended. Commercial preparations for raising alkalinity are balanced mixtures of sodium carbonate and sodium borate. in the right ratios, they can adjust alkaline reserve without suddenly raising ph. Baking soda alone will rapidly raise the ph and could stress the inhabitants if raised to fast or too much.\r\n\r\nAnions such as phosphates, silicates, borates, hydroxide, carbonate and bicarbonate work together to produce alkalinity. In the ocean the constant influx of clay into the water from coastal rivers provides the alkalinity that buffers oceanic ph. in aquariums bicarbonate and carbonate provide the alkalinity to stabilize ph. (do not use clay in the aquarium). think of alkalinity as acid neutralizing capacity.\r\n\r\nThe major polyvalent metals that determine sea water hardness are calcium (Ca++) and magnesium (Mg++).\r\n\r\nHardness does not determine ph or alkalinity however, in healthy seawater systems a high hardness is usually associated with a high alkalinity and with a high ph. It is not unusual to have a high calcium concentration but an inadequate alkalinity that will prevent coraline algae and hard corals from taking up calcium for growth.