If you START with a couple of gallons (NOT just before using it but as soon as you dump the tank). The MAIN way you prevent clogs as you experienced previously though has little to do with how much water you begin with but more about how much water you USE when flushing. Even if you start with five gallons you still need to use plenty of water when flushing. Skimping on water is a sure path to clogs. EVER leaving the black tank completely empty (in storage or in use) is another way to create odors and clogs. Keep water in it 100% of the time.