Hydroxide itself is a perfectly fine alkalinity supplier. 0.1 mM NaOH has 0.1 meq/L of alkalinity even when measured directly with nothing else happening.
But when added to seawater, this happens:
What Your Grandmother Never Told You About Lime by Randy Holmes-Farley - Reefkeeping.com
What Is Limewater?
Aquarists have used limewater very successfully for a number of years, and it is the system that I use in my aquarium. It is comprised of an aqueous solution of calcium and hydroxide ions that can be made by dissolving either quicklime or lime in fresh water. Note that the water must be freshwater. Combining lime with seawater will result in a mess of precipitated magnesium and calcium carbonates and hydroxides.
The only inherent difference between calcium oxide and calcium hydroxide is that adding a molecule of water to quicklime produces lime, and that a great quantity of heat can be generated when that happens.
3. CaO + H2O --> Ca(OH)2
Quicklime + Water --> Lime
Consequently, dissolving quicklime can make water quite warm, especially if an excess of solids is added. Some aquarists have damaged equipment by adding a large amount of quicklime to a small amount of water in a plastic reactor. The heat released can easily boil the water, and some plastic devices may not be able to withstand that hot, corrosive mixture.
The calcium ions in the solution obviously supply calcium to the aquarium, and the hydroxide ions supply alkalinity. Hydroxide itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when limewater is used in a reef aquarium, it quickly combines with atmospheric and dissolved carbon dioxide and bicarbonate to form bicarbonate and carbonate:
4. OH- + CO2 --> HCO3-
5. OH- + HCO3- --> CO3-- + H2O
In an aquarium with an acceptable pH, there is no concern that the alkalinity provided by limewater is any different from any other carbonate alkalinity supplement. The hydroxide immediately disappears into the bicarbonate/carbonate system. In other words, the amount of hydroxide present in aquarium water is really a function of only pH (regardless of what has been added), and at any pH below 9, it is an insignificant factor in alkalinity tests (much less than 0.1 meq/L). Consequently, the fact that alkalinity is initially supplied as hydroxide is not to be viewed as problematic, except as it impacts pH (see below).
Limewater that is saturated with calcium hydroxide has a pH of 12.54 at 25ºC. It is actually recognized as a secondary pH standard. The pH is substantially higher at lower temperature (12.627 at 20ºC and 13.00 at 10ºC), and lower at higher temperature (12.289 at 30ºC; 11.984 at 40ºC). Saturated limewater has a conductivity of about 10.3 mS/cm at 25ºC, and contains about 808 ppm of calcium and 40.8 meq/l of alkalinity. Slightly more calcium and alkalinity dissolve at lower temperatures, and less at higher temperatures. Of interest to chemists, a large fraction of the calcium in saturated limewater is present as the ion CaOH+, with the remainder being Ca++. The CaOH+ will instantly dissociate into Ca++ and OH- upon its addition to aquarium water.
