If your plants are thriving and utilizing the wastes in your tank there
may be a decreased need to suction your gravel. You may want to
reseach planted tank as it relates to cleaning/suctioning.
The way to avoid making a crater in your substrate is to place a plate
or similiar flat item(that has never seen soap) in your tank
temporairally. while you are filling the tank. Then pour your water
slowly onto the plate.