You should clean your biorb every couple of weeks, or more frequently if you notice your water is dirty or water levels are unhealthy. If the biorb filter sponge gets dirty before a change is due it can be washed.
If you have a 20-gallon tank, you could choose between the 10-20 gallon filter and the 20-30 gallon filter. Gravel in the tank helps maintain healthy bacteria in the water. Bacteria may sound undesirable, but the bacterial colonies that dwell in fish tanks actually help balance the pH in the water to the level that your fish need in order to thrive. To clean your tank, take out all of the decorations and clean them with a toothbrush and warm water.