Well if your calcium and alkalinity go a bit out of whack then you could end up with what's known as a "snowstorm" in your tank.
Here are some excerpts from WWM and advanced aquarist...
In troubled systems, the misapplication of calcium supplements (dosing suddenly or to excess) is known to cause a sudden precipitation of carbonates (the alkalinity falls/crashes) that is commonly referred to as a “snowstorm”. It is instigated by the influx of a large or rapid amount of calcium entering the system that spikes the pH immediately surrounding carbonate molecules and causes a crystalline precipitation (fallout).
To safely avoid dangerous imbalances in the Ca-Alk dynamic, aquarists simply need to avoid pushing either component to an extreme end or both simultaneously high. Instead, think of the relationship as a Hi-Lo situation within the safe ranges. Within the accepted ranges (350-450 ppm Ca and 8-12 dKH Alk), one parameter can be pushed to a high end while the other is allowed to stray toward the middle or lower end. Any reasonably mix of the two will still provide more than enough of both elements for successful calcification. More importantly, consistent levels of both are far more supportive of growth in calcareous organisms than the inconsistent but high average of either component otherwise. Many aquarists enjoy phenomenal growth in their reef creatures with rather modest Ca and Alk levels. Indeed, consistency with all aspects of aquatic husbandry is more conducive to success than random high points.
Consequently, limewater overdose, especially dosing of lime solids, is by far the most frequent cause of “snowstorm” events where calcium carbonate precipitates out all through the water column. In some cases, the tank can look like milk. The good news is that this event rarely causes lasting harm to tank inhabitants (at least that has been recorded to date), but it is nearly always upsetting to the aquarist.