It is the soluble calcium hydroxide which causes the high pH, so if this can be stabilised the concrete is 'cured'.
Contact with carbon dioxide (in air or water) insolubilises the calcium on the outside as carbonate, just takes time, and seals any strong alkali in the centre of the 'rock' where it cannot affect the aquarium if the rock is not broken.
Sodium silicate (waterglass) solution used to be used to cure concrete pools, making insoluble calcium silicates to help seal the surface (instead of carbonate), but the caustic soda produced in solution still had to be washed away.
Both the above techniques add to the surface. Attempts to neutralise the alkali using acids tends to strip the surface off the 'rock', exposing the aggregate which reduces the surface area of cement able to supply alkali.