Hi Lindy! In low tech tanks such as ours were we are not flooding the tank with water column fertilisers (to keep your TDS low for the chocco's/paros), some decomposition in the substrate is good thing. This decomposition provides nutrients and CO2 for the plants, which in turn helps keep the tank healthy. My tank has been running for about six months to date, and i have only given the substrate one very light clean (recently).
Hope that help?