[inwall75]

Yes, this will work but you need an airtight lid. Evaporation of the slurry can cause the calcium carbonate to fall out of the solution. This is called the slurry method.

As you may be aware, surface seawater is supersaturated with respect to calcium carbonate, so calcium carbonate is the first mineral that forms from evaporating seawater. Even slight evaporation of seawater in the presence of calcium carbonate crystals is sufficient to form “whitings” or spontaneous calcium carbonate “snowstorms” — such as large banks of oolitic sand on the Great Bahama Bank and areas where rapid evaporation of shoal waters occurs (Morse and Mackensie 1990). A freshly mixed batch of one of the many types of synthetic seawater will also be supersaturated with respect to calcium carbonate. As those of you with reef tanks know, calcium carbonate does not redissolve rapidly under aquarium conditions, so any calcium carbonate lost in the process of mixing synthetic seawater will effectively be permanently lost.

As seawater continues to dry, with between 10 to 25 percent of the water remaining, gypsum is formed (Millero and Sohn 1992). This is a process of geophysical significance. The gypsum sands of White Sands National Monument (in New Mexico) were ultimately derived from the evaporation of an ancient sea, and then the subsequent dissolution of that calcium sulfate and its reprecipitation and dispersment from a drying inland lake (which had no river outlets to feed it).

http://web.archive.org/web/200112172...io/default.asp

Why does it matter if you end adding undissolved CaCO3? Here's why.

Some have suggested that these particulate products (whether dry or as a slurry in water) can be added directly to an aquarium to provide calcium and alkalinity. Unfortunately, that method does not work well. Calcium carbonate is already substantially supersaturated in seawater and in reasonably maintained reef tank. Consequently, adding more solid does not lead to dissolution. On the contrary, since the water is already supersaturated, addition of solid calcium carbonate can actually lead to a decline in calcium, magnesium, alkalinity, and pH.

http://www.advancedaquarist.com/issu...y2002/chem.htm