Your septic system is designed to be full. When you flush the toilet, the waste water goes into the tank where solids can settle to the bottom and bacteria can break things down. However, this takes time and, since the tank is always full, one flush into the tank means one flush out of the tank on the other side. If you're using too much water, the waste water that's coming out will not have had enough time in the tanks.
The moral of the story: reducing how much water goes down your drains is one of the best, easiest and cheapest ways you can improve the functioning of your septic system.
Visit the CHA Shop to get your copy of our Reduce Water Use Tips sheet.