Do you replace/add more substrate over time?

From a chemical reaction standpoint, I believe that it would take a large amount of time for any element to be fully released and not able to be re-captured by any substrate. Similar to how many use those (in freshwater) planted tank substrates such as Flourite which will release and soak up specific chemicals.

I do always try to keep a fine layer around the whole bottom, and dependent on species I will add more in specific areas to try to have them be more comfortable by creating hills in the sand as if it was done naturally by the tide/current.