A slurry is like a thin watered down mix of toothpaste and water, it is a suspension of undissolved Ca(OH)<SUB>2</SUB> in water.
For small tanks it is better to use the saturated SOLUTION of kalkwasser (the clear part once the solids settle after heavy agitation) as topoff, as not only does it provide the calcium and the DIC (carbonate/hydrocarbonate or alkalinity once the hydroxyl driven reactions with dissolved CO<SUB>2</SUB> occur in the water column) for alkalinity in equimolar amounts for reefbuilding organisms, but it also maintains a constant salinity, as it is usually provided as a automatic top off vehicle or poured in as batches for topping up tanks. With the slurry, the biggest detail is to provide the slurry into a high flow area, preferrably one free of specimens (like your sump at the overflow drains to the sump) to allow for solubilization of the particulates prior to distribution in the tank. This prevents "burning" of a specimen due to the accidental landing of a particle of CaOH<SUB>2</SUB> on a coral's tissue.
I like Randy's work and most of the protocols he promotes, but I am not a big fan of using the dowflake either, simply because there is no assurance of contamination-free CaCl<SUB>2</SUB> in the Dowflake product, after all, it IS intended for the sidewalk or the driveway...

Look at Pickle Crisp (by Ball pickling company) for a food-grade source of CaCl<SUB>2</SUB> if you prefer using a 2-part supplement, just remember to maintain equimolar balance during supplementation of all your hermatypic supplements. Keep in mind that 2-part supplements tend to skew your conservative element proportions with excess Na and Cl over time, so water changes become mandatory on a regular basis . Continuous use of Kalk has a tendency to drop Mg over time as well (as precipitated MgCO<SUB>3</SUB>) which would be aggravated by the use of slurry-sourced Calcium Hydroxide (there are now particles in the water that can act as seed crystals for precipitation of supersaturates in high pH localized microenvironments...)
ceige said:
my "temp" system is about 50 gal....will 20ml be overkill for me?
Most likely not, but it will depend on how many specimens you have and how quickly your corals use calcium, your photoperiod, and the starting concentration and Ca<SUP>++</SUP>/Alkalinity balance of the water column in question. See one of the reef calculators for the actual numbers you need to use.
HTH, and thanks to Dong for supporting a local chemistry forum! So REALLY NICE to see a local club with a chemistry forum!