microwaving and boiling rock does more harm then good. they will both kill things in/on the rock but the nutrients are still left behind to rot and foul the water. if you want to start over from scratch with this rock and kill everything/eat away all organics, fill a tub with enough water to cover the rock and add 1/2cup to 1 cup of bleach to it and let it set for 24 to 48 hours then rinse 2 or 3 times with water and let air dry for 48 hours