I have a 265...up to 60 gallpn water changes into septic. I have had 40, 72 and 93 gallon tanks all running at the same time and never had an issue with septic in 15 yrs. Maybe I have been lucky. I had my septic pumped last year for the first time in 10 yrs. The guy was impressed...But I imagine a lot depends on the other stuff you are putting in. A 40 or 60 gallon WC is followed by ro/di flush at least 2 fold because I am remaking water right after WC....and the normal household use would dilute several fold. I have a 3 bed ISDS almost 35 years old. If you can do it another way I would say do so but I stopped losing sleep over it