Jaws- I'm all too familiar with living with a septic system and had to replace the entire system on our last home (before we sold it). Our current home is on a 40+ year old septic system with a 1000g tank and leach field system. If you don't know the age and/or condition of your system I would absolutely not inundate it with waste water all at once like you will from large water changes from large tank. Even if my system was brand-new I would still dump water change water outside and away from my sand filter / absorption field. I pump my waste water out the door and onto the lawn and flower gardens away from my septic.
The life of a septic system is directly proportional to how much water is processed through it. Things that extend life is pumping the non-digestibles out of it every few years, cleaning the zaebo filter if so equipped, being very mindful of what you flush and put down sinks and being smart with how much goes down your drains. SO gross, but don't let the ladies of the house flush tampons. Ask me how I know this.. UGH... NO flushable wipes either! Don't use real fluffy toilet paper either. You'll have a mass of gross paper pulp in the tank if you do.