I actually just finished the chapter in my reef chemistry book that goes over this.
It says that you will see a decrease in alk, as aquariums age the corals and coralline algae grow which increases the demand for calcium and carbonate alk. Detritus occumulation in the substrate , in time bottom substrata collect detritus that impedes the advective flow of water, decreasing the o2. Phosphate occumulation, not usually a problem, but can happen. Heavy metal accumulation, accumulation of allelopathic coral metabolites. Flow reduction in old pumps, lights...etc etc. ALl contributes. I think is says something like replacing the LR and sand every 3-5 years. I would have to double check to be sure.
Good point MT79.